More important than what it says on the door, or what anybody else does . . . what does it say on the tires is the maximum pressure? If it's Load Range E, it probably says 80 psi; if it's Load Range D it probably says 65 psi. If the tire says less than that, it's probably not the right tire for your truck. Those are the MAXIMUM cold pressure ratings. If you are towing or hauling then you probably want to be at or near the maximum. If driving around otherwise, something less will soften the ride, improve handling, and help wear evenly. How much less? Watch for tire
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 vaguenes
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),
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.
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