Have you tried putting the rear axle on stands and bringing the RPMs up to see if it is indeed the rear end, of something in the transmission, of engine
Get the rear on jack stands and put it in drive, then steadily increase the rpm till it shifts the do it more till it goes into 3rd. Then increase till your at 65 mph.
Remember to use the brake till both wheels are at a full stop before you put it in park if it's an automatic.