hippydave, I don't think you'll find any specs for the amount of "slop" allowed. My truck has three driveshafts due to the long wheelbase, both of my midship bearings were loose in the rubber isolator, the bearings themselves turned smooth and quiet. I disassembled the shell and used some old rubber tarp straps to shim the existing rubber and take up the excess looseness. If the bearing itself is rough and has ANY play in it, it's time to replace it. If it's just the rubber, use your imagination and whatever you have on hand to tighten it up, too much play at the midship bearing will show up as driveshaft vibration.
