86 GT convertible 5.0 w/gt40 heads

Just changed rear gear from 2.73 to 3.73 and getting high speed drivetrain vibrations.
Checked tire balance,wheel run out and they are good.

I have read that the aluminum driveshaft is usuallly the solution.

I prefer the Ford motorsport one but they are currently are 10 week back order.

Does anyone have experience with the Summitt [h=Summit Racing SUM-790300 ? or anyother aftermarket ?]2[/h]Thanks