If it is the axle pinion yoke nut. Then the pinion can move around chewing up the reared or letting the yoke walk around. Yoke could come apart and drop the drive shaft on the ground and maybe lock up the rear end.
If it's off the transmission it here again could drop the drive shaft and/or chew up splines on the yoke or shaft.
I'd be getting it figured out sooner than later.