I have a 98 F150, 4.6, 4x4, 3.55 gears NON-LS. I bought the truck about a month ago, the guy i bought it from let it sit for 8 months. ive been driving it for a while and today the rear end of the truck started bouncing when i was driving in town, i never got over 45mph. and it does this when braking too, but stops after i apply harder pressure on the brake pedal.

what could this be?

 

I would suspect sticky rear brakes/parking brake.

 

i have drums on the rear, and i replaced the wheel cylinder a while back, and i adjusted the brakes with the star thing, and it hasn't gave me any problems until now.

and when i took the drum off to do that, the parking brake cable fell on the ground, it was just sitting in there. it had rusted off, but it didn't hop until now.

 

Check for drag now.

 

Bad shock? Knot in the tire?