Bought a used 2003 F150 4X4 Heritage Edition about one month ago. Thing has been working terrifically. Yesterday, I took it for the first time off road. nothing crazy. Basically a dirt road in 4 wheel drive for about 5 minutes. Not much more extreme than a paved road really. Then, as I am driving home, any time the truck hits about 40 MPH, it starts bouncing up and down pretty violently (like and inch or 2 to the bounce at a rate of about 2 - 3 bounces per second). Felt similiar to driving on the rumble strips they put on the side of the highways. I slowed it down to less than 40 mph and the bouncing stops. Speed up to 40 mph, it starts again. I pull the truck over and i notice smoke coming from front passenger side brake system. But see no brake fluid leaking onto any hot parts. I felt around all tires and I feel no bumps or protrusions in any of the tires and they are in pretty good shape. Any ideas on what is going on here?

 

Did you stay in 4wd high. Sounds like something to do with 4wd system.

 

i experienced the problems after i put it back in 2 wd