The 5-speed Allison has PRND321 on the shifter. Press/hold the T/H button for a few seconds and you should get a D-with-slash on the cluster display, aka overdrive lockout. Press/hold the T/H button for a few seconds to re-engage 5th. They used this instead of a "4" on the shifter. When you count shifts it may feel like it's a 6-speed, but one of them is the torque converter locking up.
What speed is the 5-4 shift? Are you in Tow/Haul mode or normal? If not in T/H mode then it sounded like just the torque convert relocking. I am only experienced with the 5-speed Allison with a Duramax, but in non-T/H mode the lockup is right around 48 mph (higher if under load).
For many years people thought the problem was the intermediate shaft. It seemed partly to blame for the clunk. The current consensus is that it's the plastic bearing on the steering shaft, under the dash. There were descriptions of an improved green plastic bearing. If you've read up on it, you've also run into zip ties, hose clamps, and muffler clamps that pull the shaft to one side instead of rattling around. I think I put a zip tie on mine years ago and since then, I just ignore any clunks. I accept it as "a GM thing". Sorry, I have nothing to say about vagueness. A couple of years ago I had a steering box put in, but that was because it was leaking. Other than that I haven't needed any front end work, except shocks and a heavier-duty sway bar, and I adjusted toe to eliminate some edge-wear on the tires.
I'm not positive about our trucks, but for most (all?) vehicles just disconnect the light sensor on top of the dash. Yes, I know "just disconnect" might not be the easiest thing to do . . . unless you're willing to burn your bridges and destroy it from above.
With 4wd, you need to be aware of the "pump rub" issue in the magnesium tail section of the transfer case. It can cause a pinhole in the tail section, very slowly leak out the ATF (in the transfer case), and ruin it. There are various pump rub kits, or if a tail section is ruined there are aluminum replacements. I don't live in a cold climate, so I'm certainly no expert about winter prep. Bio-diesel is very good for the motor but gels much quicker in cold weather. Avoid it in the winter if you can. You'll have to study up on anti-gel additives (like Diesel Kleen 911, I think), and using Diesel #1. The glow plugs and controller need to work well in cold weather. Turn key on, wait for glow plug light to go off (should be about 5-10 seconds in cold weather, I think). I always suggest a lift pump and more/bigger fuel filtering and water separation. I use a Kennedy Diesel lift pump and Donaldson filter on a Nicktane auxiliary filter head. Beware that some setups flow too much and cause problems with the return into the tank, so be sure to read reviews on the so-called "bigger, better pumps". I assume you know that the LB7's had problems with the injectors. Again, there's lots to read on that, too. Again, I'm lacking experience with the LB7 since I have an LLY with it's own list of potential problems!!
While it costs more than rebuilding, many have had better results with just replacing with a new unit. I replaced mine with one made for 2011 (ACDelco 178-0853). It may require a little bit of filing on the rod eyelet so the brake light switch works properly, but there was also a significant improvement in my brakes. Read reviews for that by others, not just me (Amazon and elsewhere).
All of the above . . . plus try new bulbs. Sometimes when they "burn out" the filament jumps over to another filament and strange things happen. Sometimes you can't tell just by looking at the bulb, either.
Blend door actuators? Notorious for going bad. I just replaced one, but it was the easy one which just put heat to the driver side all the time. Rat nest? Other debris like leaves in the vents? But sorry, I don't know where to start looking...
Basically, the "something special" you need is ... a Duramax pickup truck. Then move whatever it is that you really like from your old truck to the Duramax truck (if that something is compatible). Maybe you can get davester to help you He already knows how to convert from 2wd to 4wd!!
Almost always it's a loss of prime. Try priming it (hopefully you know how). If the filter head hasn't been rebuilt or replaced in the last few years, it's probably leaking air. Also, the soft fuel lines are getting old and leaky. Sometimes even the hard lines are rusting, and sometimes from the inside out. If you don't have a lift pump, the pump on the engine sucks fuel all the way from the tank and through the filter. A "leak" anywhere will cause a loss of prime. I say "leak" in quotes because air leaks in and fuel does not (usually) leak out at all.
Certainly sounds like a stepper motor. While a more common problem in the older trucks (I replaced all of mine years ago), it seems like any stepper motor can go bad. If you have something that plugs into the OBDII port and displays stuff, you could watch the speedometer speeds shown there. If they look good there, then it suggests the stepper motor for sure.
As long as whatever you are doing doesn't cause jerks or clunks, then it's fine. By that, I mean (for a Duramax, but I'm sure similar for you), for me, between about 28-48 mph the torque converter is locked up in T/H, but unlocked if not in T/H (generally). If I am under slight acceleration/load and switch out of T/H in that range, the drivetrain gets a jerk/clunk. So I don't do that. I don't think it's as touchy engaging T/H in that speed range, but still I make sure there's no load so the transition is as gentle as possible.
Of course!! [On edit] Ok, maybe I should qualify that a little. You probably shouldn't shift to a lower gear that would cause the engine RPMs to go above redline. But I also think the computers are going to protect the engine from doing that anyway; still, why test it? I guess you shouldn't be shifting it every 2 seconds, either, but I don't think that was your question.
2004 was a transition year from 1st gen (LB7) to 2nd gen (LLY). Look at the RPO codes in the glove box to see. Each has their own distinct list of issues. Maybe the biggest for each: LB7 - injectors (1st set were probably replaced under warranty). LLY - overheating (towing up long grades, especially in the west); injector connectors (#2 and #7 notorious; but the injectors hold up very well!); headgaskets (especially if overheated or had tuner). I usually recommend the LLY (I have one) only for people who are likely to work on it themselves (maintenance, looking at known potential problem areas). Regardless, if it's had a tuner I would walk away. I hate that it sounds like I am trying to scare anyone away from these trucks. I've had mine since new and plan to keep it "forever" because it's a GREAT truck (but I've taken steps to prevent overheating, worked on the #2 and #7 connectors, protected the known wire rub-thru spots, etc.).
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