i was having the same problem with a 65 dodge a990 clone i built for a guy, i tried everything 3 times, pinion angle up,down,2 and 4 degree's, new driveshaft balanced and rebalalanced, axles checked, wheels and tires, pulled the trans twice and had direct drive drum changed, called schumacher, prayed a couple of times, i could go on, still doing the same thing, it finally broke me and i gave up, the owner is suppose to be taking it to oceanside driveline to see if they can figure it out .