is it a noise (rattle) or a physical vibration? noise may be lack of grease whereas the vibration and/or noise could signal worn parts.
get underneath the truck and grab the driveline at the u-joints. put a hand on either yoke and then turn your hands in opposing directions, back and forth -- if you feel slop or play, your joints are probably toast. do the same with the slip joint -- there shouldn't be any play and if so, the splines on the driveshaft are worn. a good driveline shop can "swap" the slip joint from the front driveline for the one on the rear (which receives less wear unless you drive in 4H a lot), rebalance both shafts, and pop new ujoints in for you.
replacing ujoints is not mechanically challenging but it is a major PITA and time consuming without the right tools. an arbor press with a large throat opening is extremely helpful but the job can be done with a hand held sledge hammer, impact sockets, a pair of Vise Grips, and a good buddy to hold the driveshaft while you knock the old bearings out. the field service manual in conjunction with the Haynes manual, does a good job of describing the proceedure.
you might also check the status of your pinion nut on the rear differential -- see if it's loose as this can cause vibration/rattle/noise too
HTH,
-dogboy- '87 FJ60