Rear Tires Suddenly Lock Up Going 65

Well despite the cold weather I took the tires off to find nothing wrong with the brakes except all the shoes had cracks but no loose hardware, nothing. Pull the driveshaft, pinion seems to have zero lateral play. Pull the axles, pull the center section, no broken teeth, no cracks, no missing parts or hardware, no fresh scars, spins nice, no binding, pinion nut is tight. Teeth seem to be broken in nice and seen some miles but nothing crazy. With the sure grip it's hard to see the spider gears but everything seems to be in the right place and undamaged. I guess it's something in the tranny.. Still confused on how it happened though.

The sure grip is a cone type and the axle is a 489 unit if that changes anything to the diagnosis of rear end or tranny