You can change both rod and main bearings with the engine still in the truck. Once in a great while that will help. Usually the problem is deeper, like a damaged crank. On my 51 half ton, I pulled the rod bearings and put new rings on the pistons. The rod bearings were beat down to the copper, and I suspected a flat rod journal or two. Sure enough, #4 was .002 flat, and will talk pretty good even with 20W-50 oil at high temperatures.
Pull the pan and look at the bearings ONE AT A TIME!!! I would recommend replacing both rods and mains at the same time. Look the crank over good as well as your old bearings. You can use plastigage to get an idea of bearing clearances (.001-.003) on the mains. Look at the stampings on the back side of the bearings. There will be a date code (mo/yr) and some part numbers. If any of the numbers indicate undersize (.002, .010, .030 max) the crank has been ground and undersize bearings are needed.
Also, the rear main bearing cap contains the seal, which can be difficult to change sucessfully in the vehicle. Get a GM shop manual for the vintage of engine you have and go for it.
