I know this is beating a dead horse but hopefully I get a few suggestions.

I have a very light rear end clunk. I can shift through all the gears ( if done just right) and not have it happen. But in city traffic I hear it the most in the lower gears. On the open road not so much.

It "feels" very light. You actually hear it more than you feel it. But even the sound is fairly light.

The CV's are good, the drive shaft and U-joints feel fine. The rear diff mount looks old but appears to be in tact.

Is there a way to know if it's the rear mount ( is there something I can do to test it) or should I just replace it and go from there.

Where is the best place to order one and what type or brand should I get.

1980 non turbo 5 speed manual 199,000 miles.

 

There have been innumerable threads about the clunk and the general consensus is that the dif mount should be replaced. Just go with a stock replacement. Try your local auto parts store. If they don't have it, head to MSA or Black Dragon

 

its most likely the mount, that said there are the mounts on the main x member that often gets overlooked. mine was the half shaft u joints as well. the only proper way to test them is to remove the shaft and check for play