I own a 1999 Jeep Wrangler and this is my problem,

You aren't supposed to shift into 4x4 while moving. You aren't supposed to operate in 4x4 on pavement, or at high speeds. I imagine you are experiencing binding in your driveline, eventually it will break like that.

you can shift into 4x4 high while going but i would recommed going only about 10mph tops. you must be stopped to put it in 4x4 low. dont go over 10 mph in low and for high dont go over 40mph to 45mph