Have a look under the car at the 4wd shift lever, there should be a "pattern" around the bottom of the lever that holds it in 2hi, 4hi and an even bigger stopper to stop it coming out of 4low. Which one is it popping out of?
depends. gota have a look for that locator i mentioned, if you cant find it then that could be ur problem, easy fix it just bolts on. if its there then it could be an internal problem in the case with the selector.