Don't know if my experience can relate but maybe some one will find it interesting.

I had a F150 that was doing exact same thing.
Replaced every thing we could think of. Spent about 1000 dollars working on it. Finally a mechanic that I had not used before took my brake booster off the master cylinder and adjusted the linkage inbetween them. Put booster bake on and the truck never did it again. He didn't charge me a dime.

Some how the booster was causing a slight rear wheel drag after I reach a certain speed or temp. This resulted in the rearend bouncing and shaking all over the road. It was wearing my brakes out faster but not so that i could really notice.

Strange,,,,,i know.

I'm slow, but consistent........consistently slow!

Loy, 1950 Chevy 3600 3/4-Ton

http://s522.photobucket.com/user/Shellology/library/1950%20Chevy%20Truck?sort=3&page=1