davester

The idler arm has neither 3 or 4 grooves, so you can't really go wrong with that one, it has a taper for the 'middle' connection between the idler arm and the idler arm bracket assembly. The pitman arm SHOULD be 3 groove for your 2500, but if you have the money and don't have a second vehicle, I would get both and then return the one you don't use.

shift lever moves the wiring harness in the steering column a bit, making/breaking the connection?

Weird. My '04 Sierra 3500 can switch on the fly, even at highway speed. And it actually switches gear, turning it on will generally make it downshift unless I'm going downhill, and then turning it off will shift back, at highway speeds.

Thx for listing the conditions the DRL's are on for, as I was incorrectly assuming they would be on with the engine [and was checking with it in Park, so no power to either plug, but fuse and relay were good, so I was about try digging into the wiring harness]. Thankfully, I googled for DRLs not working and came across this thread. Now, I put on the parking brake, put it in gear, and like magic, power to the DRLs. One DRL was working fine, and the other has an iffy socket [there seems to be a bunch of burnt resin in the socket and both sockets and blubs look like they've been getting too hot]. I might have to go to the men's mall and get some replacement sockets, but it seems like they weren't designed right [as the bulbs should be able to be on all day without overheating the socket].

I believe the manual says that if you don't get a 'change oil now' alert within 12 months of your last oil change, you should change it [presumably a time limit so that if the computer has a software failure, you still change the oil].

I'm not sure about your specific truck, but my '04 Sierra 3500 tracks how you drive [lots of stops/starts vs lots of highway driving, heavy acceleration, lots of idling, etc] to compute how much life the oil has left. Does your truck have that?