I bought my truck used last September. It's seemed to have this horrible "bounce" especially around the 40-45mph range. It's very inconsistent. At times it will drive like a cadillac and at others it will feel like I'm about to get launched out of my seat and hit my head on the roof. It's like my entire frame is flexing in the middle. The dealership said my tires were out of balance. They rebalanced them but it didn't help. I'm thinking I might have a tire out of round... I don't know. Also, whenever I hit a bump at highway speeds, the truck seems to take a while to recover--it shimmies for a while before it smooths out. I believe my rear shocks are shot. They have rusted leak-down marks on the outside of them. I'm going to replace them but I'm not sure that shocks would cause that big of a problem. One thing that has been consistent is I noticed that never have any problems when I either have a load in the bed or am towing a heavy trailer--anything that can put a significant load on the rear of the truck. The bounce always seems to come from the rear or right under my seat. I don't have any wobble or vibration in my steering wheel. Also, I've rotated the tires. That seemed to help for a little while but now it's back to the same. Any thoughts?