Before you get too involved check the transmission mount. The rubber cushion deteriorates with age and oil contamination and will create a vibration in the speed range you have yours. Just remove the 1 or 2 bolts that hold it to the crossmember pad and raise the tailshaft up with a floor jack. If the mount falls apart in 2 pieces or can be easily twisted and moved by hand while still attached to the tranny it is gone. No more than a $20 item and 15 minutes to replace it.
