Been searching threads to find something similar, no luck as of now, ...with any throttle input, (in 4high or 4low) both the r/f and the r/r begin to spin and you are done. Very delicate line between keeping 4low pulling and any increase in throttle input? Only dealing with X amount of inches of snow not crawling or mudding.?. Went through a spell this winter of the fnt diff engaging but think that was from lack of use.?. now it seems to shift from 2 - 4hi - 4low without problem. Nature of the beast or locker issuses? Never had to use the 4x4 in any of my S series for more than play. This winter they are being used everyday since Christmas. All thoughts and inputs will be greatly welcomed!

 

Sounds like the typical open differential issue. With any 4wd that has an open differential in both the front and back, you will get this type of behavior (only one wheel on each axle spinning).