2001 Nissan Frontier Rough Shifting

I have a Nissan truck with about 108,000 miles, automatic transmission, with a four cylinder engine. I feel a pretty good bump in the rear when the gears shift, usually only in shifting from 1st to 2nd or at low speeds. It feels like something is giving way under the torque of the shifting motion. I have replaced the rear shocks and inspected the u-bolts holding the leaf springs to the rear axle. There seems to be excessive movement at the juncture (u-joints) where the drive shaft meets the rear end gear housing. The whole rear axle assembly seems to be rocking back and forth. I can see this when I rock the truck back and forth by pushing on the rear tires. I would love any advice as to what might be the problem here.