Hello everyone. I'm new to the forum, but not new to this mustang or pinion angle/driveshaft problems and thought I'd share some of my findings. I'm no expert by any means; just a regular joe.
Anyway, I've had two different shafts. A Shaftmasters and now a Powerhouse. The Shaftmasters shaft just didn't work out. Period. Noticable vibes at 90mph; car felt like it was coming apart at 120mph. Adjusted pinion angle repeatedly from -2* to 0*; nothing worked.
EDIT: I did buy the shaft secondhand and the guy told me it was from Shaftmasters. They did work with me to resolve the problem and offered to rebalance the shaft. Basically, I'm not trying to say that they have a bad product, but that this particular shaft option just didn't work out for me.
Enter the Powerhouse. This time, while there was no shaft in the car, I took the measurement from the transmission output flange and the pinion flange to get the angle that I wanted (-1*). Car is smooth as a baby's butt now. Could have been the shaft (most likely) could have been the angle (probably not).
In browsing this thread, I noticed some things that I encountered that weren't mentioned.
-If you have LCA relocation brackets, it will affect your pinion angle. Make sure you readjust it if you have these.
-I saw that one guy had his shaft rubbing in the tunnel. There is a stamped hump in the tunnel that can cause your shaft to rub it, particularily if you are lowered. I attached pics if you don't know what I'm talking about.
-I actually wrote out which way to turn the UCA with a sharpie on the frame of the car; I kept forgetting which way did what. Unfortunately, I don't have a pic, but maybe I can get one tomorrow and post it up.
Hope I was able to help.