Thank you for the replies. My car is a 1984 El Camino that came with a 305 and a TH350C trans. When I bought the car it had a regular TH350 trans in it and a new trans mount. Someone prior to me attempted to make a driveshaft by cutting a longer one down. Their workmanship was poor as the yoke was welded off center by .050" causing a buzzing vibration at speed. I had Southwest Speed make a brand new driveshaft.
I still have a minor vibration after replacing a few brake drums, both axle shafts, all control arm bushings, trans was rebuilt with new output shaft bushing and balancing wheels and tires.
After seeing how the pad was advertised as 'TRANSMISSION CROSS MEMBER INSULATOR U 78 -87 EL CAMINO MONTE CARLO MALIBU' I began to wonder if the height difference might cause the vibration.
If not all cars came with the insulator I will have to look to other sources for the vibration. At this point I am starting to think it may be the pinion bearing or something inside of the differential. It has a 2.41 open 7.5" 10 bolt. I don't hear any obvious whining from the rear but I'm betting it has well over 100k on it at this point.