Vibration Problem

[RDMumfts Stang]

I have an '85 Gt with a vibration and am trying to narrow down where it's coming from. It has over 147k miles on the T-5 and it is getting very balky. It doesn't like going into 1st at lights without a fuss and it refuses to powershift any more. My car starts vibrating at around 70MPH. I don't believe the vibration started until after replacing the 2.73 geared 7.5" with a FMS 3.73 8.8". I make decent power (14.6 @ 96, w/2.73s, street tires, short shifting 3rd and 4th gears, 100+ degrees).
Does the t-5 vibrate when worn out? Could it be that the vibration dampner from the 7.5 not being large enough to absorb the vibration? I installed Megabite Jrs and police UCA's when replacing the RE. The added bite hit the drivetrain unlike ever before, hastening the T-5's woes. I just upgraded the brakes to '99-newer fronts and Mark VII rears and 99 17" wheels. I had hoped replacing those components would fix things. I have also had the driveshaft balanced since the vibration started. Are there any other possibilities I may be overlooking? Thanks in advance for the help.