My rear end is hoping like someone is jumping in the bed as im driving down the road. It started after going to 285's and its not the tires i have had them checked and re balanced. It only seems happens when im cruising or slowing down with the engine at idle and if i get on it a little it seems to go away. Any ideas?

Chris

 

Axle wrap, maybe? Maybe you need a set of traction bars.

 

When youre coasting and not when youre on the throttle? Ring and pinion problems? Maybe have a look at the driveshaft and transfer output. Something fishy there.

 

Is there a particular speed where this starts to happen? I ask because I encountered a driveshaft vibration in my 99, but it only happened above 75 mph. It was the u-joints, even though not a single cup was dry when I pulled them apart. New u-joints and all's smooth through 100+ mph.
Now this didn't feel like the bed was jumping, but is was a significant vibration once I hit 75+ mph.

Chris