This is usually due to a binding that happens when the rear wheels have turned at a different speed than the front due to turning left or right. the shift lever could be bent or dirty needing cleaning and lube, or you could have low fluid level in your T-Case. If you have the vacume type shifter in your front end, a line may have a leak or fallen off. The vacume unit on your front end if you have one may be bad and need either cleaning or replacement. I hope this gives you a place to start looking.