The slack you're seeing is probably right on the ragged edge of "too much". I'd suggest removing the rear cover plate of the differential and inspecting the ring and pinion gears, and especially the spider gear cross pin for slack or noticeable wear. If the rear end lube hasn't been changed recently, inspecting the gears is a good excuse to service it. If you don't see any noticeable abnormalities, the next step is to unbolt the torque tube ball pivot at the back of the transmission and inspect the universal joint. If you choose to disconnect the torque tube, be sure to support the truck on jack stands at the frame rails, not the axle tubes. Weight on the rear end housing causes some pretty big downforce on the torque tube, and the tube can slam down as the U joint is unbolted. Safety first!
Jerry
