1999 Ford Ranger 4 whl lo clunk

Hi! Can anyone help to identify the source of this clunk? I use 2nd gear, 4 whl drive lo daily going up and down a track on my property. Recently, but only periodically, I get a big metallic CLUNK going either up or down the grade. And only in 2nd lo. Have never had it happen in 1st low or 1st high, the only other 4 whl dr gears I use. I'm going very slow and it seems to always happen when speed is consistent and there is no jarring motion from dips or bumps. The track it happens on is about 1/4 mile and it might happen only once along the way and I'm able to continue on in the same gear in a normal manner. Because it happens only in 2nd lo and it does feel like the source is directly below me, I'm inclined to think it's coming from the transfer case. Perhaps some sort of a momentary skip or slip? Thanks