U-joints, torque rod bushings and maybe tailshaft bushing are all possibilities, like the others said. But after changing your trans mount you may have changed some geometry of the driveline and now you need to shim the driveshaft. There was actually a Volvo TSB about this but I can find it. You need to sandwhich some washers between the center bearing crossmember and the bearing holder. Start with shimming 1/2" and then remove washers one at a time until the vibration is gone. good luck, I sure a well-titled post could give you some info on the actual TSB procedure.

Matt

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'82 DL - 153k, '93 945 - 102k