Okay, this is probably too late for the original poster but it may help others. I bought a 2004 F150 w/ 30K on it. Drove it for two weeks and it was fine. Then I had the following happen three times, on 3 different days. Driving along I'd all of a suddne get a violent shake in the whole vehiclethat felt like the rearend was locking up. this happened when driving at 45mph, 70mph, then 10 mph. each time the problem went away by itself. The problme was actually caused by the emergency brakes!
It seems as if the e-brake is a mini drum machined into the rear brake rotor. The e-brake cables would freeze up and the brake lock on causing the rearend to shake/hop. Still can't figure out why this would happen all of a sudden while driving but lubricating the cables, remachining the drums and replacing the e-brake shoes cured the problem.