Loud clunking in 4WD

When I put my 1985 Cherokee in 4WD, I get very loud irregular bangs under the drivers side. It will then only disengage from 4WD to 2WD if I back up 30 ft or so. It is almost as if it is stuck between 4WD and 2WD. Maybe its the transfer case. Anything I can do to figure this problem out? Thanks

I'm not exactly sure if this will help but my 4wd did that too but then later it just went out and stopped working all together. Come to find out it was just some lag in the shifter fork in the front axle dissconnect.