T5 5th gear vibration
Reply #3 –
The dampener was installed to add mass to the rear end and eliminate an objectionable howl that is particularly noticeable in 5th gear on some Mustangs with 8.8's. Not a vibration per se, but an acoustic annoyance nonetheless. My 2006 Mustang GT had it bad in 5th gear. Sounded like a bad ring and pinion. Ford issued a TSB on it since there were so many complaints.
I had an F150 once with a 4 spd overdrive manual that developed a vibration in 4th gear that felt exactly like tire balance, but it wasn't. Traced it down to a recently installed TRW U joint. Replaced the joint with a Spicer, rebalanced the driveshaft and it was smooth as glass.
Good luck. These type of issues can eat your lunch.