Did you try what I suggested??? The car has a two piece driveshaft, with only one universal joint (in the middle)..
It has a slip joint , and center bearing/ support in the middle of the shaft also.. On either end of the shaft there is a rubber disk that connects the shaft to to the tranny, and diff...
Over time, not necessarily miles, the rubber parts deteriorate, and the grease in the bearing becomes old, and brittle.. That may well be the source of your vibration..
When I disassembled mine, it was obvious..