Do you have a manual or automatic transmission? How many miles on the vehicle?
Tell me . . .when driving at slow speeds, in heavy traffic, (10-20 mph) does the truck seem to buck when you accelerate, and then buck again as you lift off the gas? Does it feel as if there is slop in the drive train? Does this go away at higher speeds, (above 25 mph)?
Keep in mind, the drive train is made up of five major assemblys: (from the engine back they are) Clutch assy, transmission, drive shaft, rear differential (third member), axles. If you have a 4-wheel drive the list will include a transfer case. (a second "transmission" that applies engine torque to the front wheels)
The problem could be wear and lost motion anywhere along this chain of assemblies. Or, it could be the combined effects of several small problems.
In order for us to help, we need to know as much about the vehicle as possible. So. when I say, "Tell us allllllll about it." I mean tell us ALL about it.