I'm not that familiar with that rig. Is that the FTX carnage?
If freewheeling, that points to driveline. Do all 4 wheels freespin? or just front or just rears?EDIT: I just read your post again and see it was just rears. Does that use a rear drive shaft? Again, I'm not familiar with that rig so I don't know what is has for drive system.
Visually check the slipper gear and pinion to see that they haven't shredded one or the others teeth and if teeth are okay, are the 2 gears still aligned and in full mesh? Motor hasn't backed off?
Check slipper first and see if the pads are still okay and the slipper is tight as recommended by FTX. Then check the motor pinion. Its possible that the grub screw holding the pinion backed out.
If you still haven't found the issue, your going to need to start taking things apart and look for potential broken parts. Remove the slipper assembly and look for broken or sheared pins or snapped top shaft.
That's where I'd start, and if noting found, keep digging