It could be a stuck shock or a binding leaf spring but I doubt it. Try getting a friend to bounce on the rear bumber (try both sides) and see if the suspension is compressing and not just the tires. If it does your truck is probablly okay suspension wise. You can also check the air pressure in the tires for over-inflation. My truck was 6 pounds over from the dealer. Then assuming all the parts on your truck are fine, you may just be hitting the bumbs a little too fast. This will cause the back of the truck too jump out like you describe(especially in a turn). Remember there is no weight on the back end of the truck, this is why they spin out easier than cars, spin the tires easier (aka traction problems under hard acceleration), and bounce around when you hit bumps. Trucks are sprung hard from the factory so they can handle the weight of towing and hauling of materials. If you want a cushy ride that will not bounce, air bag the truck and it will be a lot smoother than the stock leaf springs. By the way, my truck feels like the back end jumps if I go over a speed bump too fast. I think it is just being a truck.