the first thing I would do right now is pull the rear ujoint caps & move the shaft all the way forward & measure the distance back to where the caps seat in the yoke.
I will check that out. About 1.75" exposed of snout and no less than 5/8 of snout is engaged in trans.
I was told you want the driveshaft to be able to slide 1" forward before it bottoms out in the trans.
There is a tool available to R&R the rear trans bushing without pulling the trans. It cost about $150. I bought one and did mine chasing a similar high speed vibration. I have yet to find it, and it only occurs above 95mph.