I have a similar issue, when i take off from a light or hit a bump with one tire i get a clunk. I found that the gov lock clutches wore down and allowed more back lash in side gears. If you don't have a gov lock you may have to keep looking
There are three oil gallery plugs on the backk of the block, these can leak and look like a rear main leak. If you exhausted all option from top of block. You may have to pull the trans or engine to locate leak.
that still be a 3/4 ton, unless you it says it on the title that it is a 1/2 ton. they made 3/4 ton six lug trucks they are just light duty. has the 1/2 ton front axle and a 14bsf rear. looks like a nice starting platform.