Play in the rear differential

[2x_Tom]

I picked up a 71 k10 4spd this morning. When I drive it I get a kind of band noise with a high pitched ping if I don't baby the gas and clutch from a stop, a shift, or letting off the gas.
My first though was U joints so I went under the truck and checked the rear drive shaft. It has play but the joints are tight. Problem is the shaft spins quite a bit, maybe an inch of rotation before it locks up. I'm thinking this is certainly my problem, the clunk then the ping from the driveshaft as it engages. Any ideas what I should check in the rear?