smythge

I have an 01 GT automatic, I am getting a vibration above 70mph it did it with the stock gears and now with 4.10s at same mph. I've tried different wheels and that didn't change it, rearend was rebuilt with new gears, and all new bearings. When the gears were being done I asked them to check and if needed replace the u-joints they said there was no play in them what so ever, so I left them.

The transmission rear seal was leaking so I replaced that a couple weeks ago, I just noticed that it is leaking again. I am begining to suspect the transmission extension bushings is bad, but it has very little play it in I can move slighty up and down. I don't want to spend $150.00 on the tailshaft extension housing if it's something else, any ideas?