1999 2500 diesel mule starts clicking intermitently when placed in 4wd. Feels like it is skipping a gear and it is a hard clunk. When engaging 4 wd, you can feel a slight vibration in the steering wheel and then the clunking starts. It will go 100 yards before it starts. Happens on soft turf as well as more firm turf so it seems it is in the drivetrain. Any help appreciated.