Probably in the front driveline/yoke/u-joints. I run into this all the time in the shop. Once-in-a-while-wheelers don't get the front joints lubed. They wear a little funny. Jack up the front and then check the front u-joint at the pumpkin. I'll bet its bad. It will feel OK unless you jack it up to test.

Tony
Owner - Maintenance Plus - (4X4 and offroad Mechanic)

80 Toy - disassembled for refitting. '78 CJ-5 still alive. 74 GMC 1-ton full time 4X4,454, B&M TH350,Detroit lockers.