Sure Grip (hopefully) question

I have what is supposed to be a early, small (741 case) pinion, sure grip under my 65 Valiant. A couple of days ago while out for a cruise I stood on it off the line and that is when it got all strange. Only the left rear broke loose. Down the road a bit I tried it again with the same result. The road was not crowned real high. Never ran in to this before. Is this a clutch style sure grip that needs a rebuild? I would doubt it is a broken axle but won't rule it out. Thoughts?

Bob