360 Balance Question

And the out come of the driveshaft inspection/balancing? Bad driveshaft/u-joints can cause premature wear of trans mount, but sounds to me like just plain old worn out original mounts, I am betting the shop you had do the job probably didn't replace them when they did the swap. Bad engine mount bushings would also expidite the failure of the trans mount bushing. You also need to take a close look at your pinion angle as that is/can and usually does cause failure of a rear u-joint (I am speaking from expierence). It is possible that just the bad engine mounts caused the trans mount to fail and in turn caused a bad pinion angle which may all be fixed by replacing the mounts but it never hurts to be sure and checking the pinion angle is relativly cheap/free if you have the right tools.