Take a look at the trans output flange, there have been a few reports of the nut working loose and flange splines stripping out. Elevate the rear wheels, parking brake off, put trans in park then try to rotate the drive shaft. You might have to use a bar to gain leverage.

Swapping the flange is easy.

To check the rear diff flange, leave trans in neutral, block the rear wheels or apply parking brake then rotate the drive shaft.

The next things to check would be trans line pressure, if the pressure regulator spring failed, line pressure would drop and not burn the fluid.

Past that, the input splines in the trans gear train may have stripped out.