Sounds like a differential problem.jack up the front end leaving one wheel on the ground ,turn key on (but don't start it) push the 4x4 to on,try to hand turn the wheel that's off the ground giving a hard back an fourth motion .see how much play is in the diff.if bad you will here that popping noise .give this a try on the back wheel as well.same way as the front .let me know if this helped .I can try to assist you with diagnosing your issue .happy an safe riding .