2001 F150 Supercrew 4x4:

I’m having a problem auto engaging 4Hi while moving. It will sometimes engage while moving slowly, but most times sounds like it partly engages, making a “gear on gear” sound. If completely stopped, both 4x4 modes engage fine. No problems engaging 4x2 from 4x4 either. Lastly, I don’t exceed the speed limit for engaging 4x4 mode.

Has anyone had this problem before and have any advice as to what may be the issue with the front driveline?

Thanks for any help you may be able to provide!