If the truck is smooth at any speed or rpm while cruising, should pretty much rule out driveshaft, U-joint, and alignment issues on the rear.... Other considerations could be a bad shock or shocks that are wrong for the application. Do you recall what weight the springs are? I've seen many instances on cars-trucks that are light in the back where the springs were way too much for the little bit of weight. I can't tell from the pic what coilovers you have, but if they're the generic 'one shock valving fits all' your problem could be there. With something light in the back, takes different valving and light springs to make it work right. Just because you can't see the tires hopping, doesn't mean that the coilovers are doing their job correctly......

As OJ mentioned, you might try adding a few hundred pounds to the rear, and along with that drop your tire pressure down to about 20 psi and try it again. Could be your coilovers are doing the hopping!!!!