I have an 85 F250 w/ a 6" Superlift...rancho 9000s. Every once in a while, at approximately 45 mph, the left rear end area begins shaking violently, almost as if the tire is bouncing off the ground. If I decelerate, it goes away. Any ideas?

 

Tire out of round or seperated. a tire store we shall go.............IMO

Had a bfg do that recently, can't complain with the miles on them

 

It might be a tire problem, but I once had a similar problem. It turned out to be a cracked u-joint yoke (on the the driveshaft). I had the same Problem you're having, a sudden violent shaking that would stop when I backed off the gas. The crack would close when there was no pressure on the drive train. It was almost imposible to see. If the tires are OK, check out the yokes and the u joints.