Axle Question

Not sure if this should be in this forum and I know it's an A body, but the principle should be the sameand no one over at FABO could help me...

What should the pinion angle be for an A body with an 8.75 rear? And how do I check this ?
I'm getting a vibration at 55 and it comes and goes like a cycle, gets faster the faster I go. I am going to have my rear tires re-balanced. But I want to check the angle to make sure.

Should point down at around 3-7 degress ( not 100% on the degress) use angle finder it has degrees on it, then check out the u-joints. My money is on the u-joints being worn out.

2.5 to 6 degress, but if the car just started doing it its not pinion angle. You need to inspect the ujoints or just replace them there cheep, check the trans mount. Also check the output shat buching in our transmission for wear. If the slip yoke is good it should not move up and down more than 10 thou inch. 99.99% the problem is in one of these three places. Summit has a pion angle finder that comes with complete instructions.