Do the vibrations come through the bottom of your seat?
If they do, this is also a a sign that the u-joints or center bearing are going south. Definitely something you don't want to put of checking, since it will likely let go at highway speeds when the u-joints/center bearing fails. Before you have the current drive line rebuilt, call a couple of your local drive line shops and see what they will charge to make you a 1pc, it is usually cheaper to have a 1pc made then to rebuild the 2pc oem ones.

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