Series lla transmission slack problem

The drive line in my Series lla has excessive transmission slack resulting in significant lurching, etc. The transmission was rebuilt by a Land Rover proffesional about 1.5 years ago (after main shaft sheared) and has no more than 2,500 miles on it. It was tight for awhile after rebuild but then it starting loosening up. The rear end, u- joints and transfer case gears appear to be in acceptable tolerances so I'm focusing on the transmission and Fairy overdrive. The main shaft nut is tight. I recently drove another Series lla and its drive line was vastly tighter than mine so its not my imagination; I'm at my wits end on this problem! :) I'd sure appreciate some advice on what could be causing this excessive and very frustrating transmission slack/looseness.
Thanks!!

Not sure if this is even possible, but is there any chance that the bolts and mounts have loosened up? Thus allowing the trannie and transfer case to move about under load? This could cause what feels like looseness I think. It would be a very quick and simple thing to check!

Brent