i recently swapped out a 8 1/4 rear end for a 9 1/4 with posi and 4.11s. I have a 727 transmission and when I bring the vehicle up to 3200 and above it vibrates like crazy, feel it in the steering wheel and when the wheels are turning every 1.5 seconds seems like a vroom vroom that's when the vehicle is at 65 mph and above. Even when I'm driving in town it seems to be vibrating a little as well. Just don't know what the problem is could it be front tires out of balance? They are fairly warn off, and I noticed a small gouge on my driveshaft. Any help would be appreciated