Check for play in the universal joints. Although a worn joint is cheap to fix, if the truck has been driven with the excessive vibration of a failed joint for any length of time, the transfer case and pinion bearings can suffer. Dents in the shafts can also cause vibrations and premature failure. Grab the shafts on either side of the slip joint and try to rotate each side in the opposite direction. If there's movement, or wose yet a "clicking" the slip joint splines are either in need of grease, or if greased are worn and will need to be replaced. Replacing the slip joint requires cutting, welding, and balancing the driveshaft. Check that the univeral joints and slip-joints have been greased, but NOT just prior to your arrival for a test drive. (There should be SOME dirt stuck to any traces of grease on the zerk-fittings or around the joints) The Constant Velocity (CV) joints used on 1980-1984 60 Series Cruisers actually have three grease nipples: one on each U-joint, and one on the alignment ball between the two joints. The third nipple is easily overlook and if it is not greased, the needle bearings in the alignment ball can wear leading to slop in this joint. Parts to rebuild the CV joints are not available from Toyota, but some driveshaft shops know where to get them.
Check to ensure that all the driveshaft nuts and bolts are tight.