I'd pull your differential cover and inspect your Trac Lok clutch retainer clips. One of the clips on my 72' came off and started allowing the clutch discs to rotate. One common thing with the clutch discs in those old Trac Lok's, when they start getting old and worn, they get loose enough to let the clips start to move around and eventually come off. They can really start tearing things up if one of the pieces get caught up in a gear.
If it's not a clip, at least by taking the cover off, you'll be able to do an inspection. The clutch kits are under $100.00, and are pretty easy to install. I'd have a qualified shop do it though. It does require a few special tool's and only should take a shop about 1/2 hour shop time if you bring them the differential.
In the picture you can see the distorted retainer clip.