If it was not swapped out then that Jeep should have a D300 in it.

I would change ALL of the driveline fluid. As Bob said, you could have a chunk of something floating around in a gear set somewhere.

Gun it and run it!
1981 CJ5,258,T-176,D300,AMC 20,Dana 30,Trxus M/T 31x10.50, Rancho 5000's, GroundPounderFab front bumper, polyethylene gas tank, aluminum dash, AutoMeter gauges

What this country needs is unemployed politicians.