Do you feel the vibration in the seats or the steering wheel? If it is in the steering wheel it is usually indicative of the wheels/tires needing to be balanced. If it is in the seat, this usually indicates a U-joint/drive shaft problem. Do you hear a tink when backing up and taking off? The tink sound is also indicative of U-joints. You can also get under the truck and see if there is any up/down-side/side play in the driveline, there shouldn't be any, you should only be able to turn it about a 1/2 inch.

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed