Make sure all 4 tires are the same diameter and dont have any excessive wear on one side..dont go by the size marked on the tires,different brands marked the same size may not always match up close enough to prevent weird handling,sizes can vary--measure them with a tape measure around the circumfrence ..
Mixing bias ply and radials can make a truck wander and pull ,and swap ends on a slippery surface,using P" rated 4 plys and "LT" rated tires with differing numbers of belts of plies can cause issues as well--ideally all 4 tires should be the same brand,tread,and number of plies..
Though I currently have tires on my 3/4 ton 4x4 that differ in both size and ply front to rear,it doesn't seem to affect it all that much..
I found the 1/2 tons anf K5's much more sensitive to tire differences ,probably because they dont weigh as much..
All the other things listed above can cause wandering --also look over the shackles on the back of the front springs,and the rear most one in the back..any slop or misalignment there can throw the axles out of alignment ,cause it to "crab walk",wander,etc..
If the front springs are sagged a lot the caster can be less than desired ,caster is what makes the wheels tend to stay straight ahead and not be deflected by bumps,etc..too little makes it squirmy..
The seized axle u-joints usually show up as a sudden pull to one side after you get it rolling about 15-20 mph..my truck did that and would nearly pull itself into the ditch till I yanked the axle and replaced the u-joints..it wasn't really seized,but one cap had zero needle bearings left and the cross nearly ate thru the cap and would have ruined the yoke had I not fixed it right away..
A crude test to see if the frame might be tweaked is to drive through a puddle and then go straight a short distance and see if the front and rear tires seem to be tracking properly..the front axle is wider than the rear on most GM 4x4's,so the front tires should stick out equally compared to the rear tires tracks if the frame is straight..