2001 ram 4x4 wont stay in 4 high. It goes in but as soon as i start moving it slips out back into 2wd. 4x4 shifter is on the floor and i notice it does seem to move/twitch a little bit under load. if i hold the shifter down in 4 high it works but pops back up into 2wd as soon as i let it go. 4 low works fine

 

Check the linkage for slop, and proper adjustment.