I've had it happen when the motor pinion teeth were rounded out just enough to slip under load as well. Fact is, the gears can look ok but are really rounded down at the tips of the teeth and loose mesh will make this happen. Try shimming behind the inside bearing of the rear diff pinion a bit so it can't move forward and backward so much and make sure the cvd cup on the other end of the pinion is slide all the way against the pinion before tightening the cup. Otherwise the pinion can move in and out away from the ring gear.