Sure-grip problem...

With as old is it is it's hard telling what sure grip unit is in there. If it is truly a 68 it will be a clutch type and if so the clutches are just worn out and are easily replaceable. If someone has swapped out the clutch type for a cone type sure grip they're not generally rebuildable but I have taken a few apart and machined down the cones to make them usable again. One was worn so bad I had to junk it. You never know until you yank it apart.

If you do have a cone type here is an article on the repair process. It's for a Pontiac rearend but it uses cones like a Mopar cone type so the process is the same except I take off .030" per cone where the article says to only take off .004~.005" per cone.

http://www.arkansaspontiacs.org/techstories/auburndiff_files/auburnfix.html

p.s. When I bought my Cuda it had a cone type and it was worn out and doing exactly what you described. I machined down the cones and sold it to a buddy and he's been using it for about 3 yrs. now and it still works fine.