what years do you think are best for full size v8 gm trucks? for buying used?
Gosh that's hard for me to say for a couple of reasons. One, everything around here rusts out so fast, pickups it seems especially, nothing is on the road for more than about 12 to 14 years on average. Another reason is I left GM and went to Chrysler in 1998 and since then I haven't been able to keep up with all the inside details surrounding the GM trucks since then.
But I see you're from Texas so maybe the trucks last longer down there. So I would have to say, and not because it's the specific years I was with GM, I really think the 1988 through 1999, or through 2001 or 2002 for the HD trucks (8600lb. GVWR and up) have a lot going for them in terms of relative simplicity, readily available and reasonably priced repair parts, no computer networks controlling everything like headlights and tail lights and wipers and climate control and blah blah...meaning you can easily diagnose and fix them with a cheap OBDII code and data reader. My '89 was a much better truck than my '01. They do have issues but overall so does everything else on the road. It seems to me when they went to the "Silverado" style trucks in the 2000's they just changed in terms of quality.
The older, what we call box-body trucks, prior to 1988 when the "new" trucks came out, are great too in my opinion. But they're just old.
I personally wouldn't recommend anything with a 700-R4/4L60E transmission. Not a bad transmission but not heavy duty enough in my opinion. The best would be an Allison transmission. Transgo used to make and probably does still make a reprogramming kit for the Allison which turns it into a seriously HD unit.
I'm afraid that's about all the advice I could give. No doubt you're looking for something newer than what I've described and I just don't have a lot of knowledge there. Oh...the 6.5 non-turbo diesels were totally gutless. The turbo 6.5s were only marginally better. I think the Isuzu (?) diesels were OK but lacked performance compared to the Fords of the day. I also think the current GM diesel are pretty good. You could really crank them up and get some serious horsepower and torque with aftermarket parts and tuning. Another thing, one problem we used to see is the transfer cases would leak mainly where the two case halves join together. Sealed at the factory with RTV sealant. Some of them like the little 231 cases in the S-10s and little Blazers held less than a quart of fluid. Nobody would ever check the fluid levels in them and they'd simply go bone dry and then self destruct. The Eaton locking rear diffs in the '87 and earlier one-tons would grenade when stuck in mud or snow...
I will tell you the truth if I was going to look for a used truck today, I'd look for a Ford. It seems today when I get the opportunity to ride in or drive someone's 10 year old GM truck they just seem clapped out and beat up.