Well guys I need some help.

Here is the whole back story.

A few years back when I grenaded my 4l60e I went ahead and replaced my u-joints at the advice of my father even though the stock ones were fine. After replacing the stock u-joints with Acdelco joints I had a vibration past 80mph. One day I got cocky and pushed through the vibration and that led to the rear u-joint coming apart at 100+ not cool. Replaced with spicer joints still had vibration.

Fast forward to when I trashed my 10-bolt. I had lived with the vibration for over a year and just never whet over 80mph. When I rebuilt the rear end I did everything. Lets start up front I have a stock 4.8l bolted to a circle d stall to a FLT lvl 4 4l60e. I had a local shop build a 4inch steel shaft with 1350 spicer joints for me to replace the suspected bad stock shaft. The rear end is just your normal 10-bolt build up: Detroit true trac, usa standard 4.56 gear set, yukon axles, yukon rebuild kit and a ta girdle, The pinion angle is spot on and I have caltracs to help keep it that way.

After the 500 mile break in I was so excited that the vibration would be gone. NOPE anything over 95mph feels like something in the driveline is coming apart. Took the shaft back to the local shop watched them re-balance and it was fine. Took all the wheels in and had them re-balanced and they all checked out. I am at a loss here. I'm suspecting trans issues now but I'm not sure that makes much sense. Sorry for the lengthy post. and thanks for any advice