the T5's I had problems with grinding in 5th was bad bearings on the intermediate shaft. Also, drive shaft phasing is critical as I had an out of phase shaft in a project that provided a nasty rattle/vibration at high speed. did you have the driveshaft length shortened at a reputable shop, or did you shorten it yourself.

also, check the output bushing.

The problems we face today can not be addressed at the same level of intelligence we were at when we created them - Albert Einstein Or with the same level of $ - Me