Check this out, I had the same thing happen to another brand truck over and over, at least 6 times. Thought Shimming diff would fix it, No. tried several fixes, even let hobby shop try to fix it, no luck. I finally noticed very little play in front drive shaft, I noticed every time I slightly hit something the gear would strip out like yours. Turns out, my chassis was bent, causing dog bone to jam the pinion to jam the ring gear. The fix, repair chassis and add chassis brace. Check the play on your drive shaft. Not the axles, the drive shaft. Just something to check. Good luck and hope you solve the problem.