I have a 05 F150 FX4 and was up on a steep snowy road yesterday, switched into 4WD with 4HI and cruised up hill - no sweat. While the 4WD was on I had to back out in one spot and noticed that the rear wheels started to spin, I also heard some grinding. The front wheels seemed to have minimal or no power.

When I shifted back to D, everything was fine again. Repeated a few times and noticed the same. I was able to get out fine but what the heck?

How can 4x4 act weird in reverse only? BTW - Reverse works fine when in 2WD, no grinding and good power, so long I'm not in 2feet of snow of course.
So, I've found stuff on the 4WD not working at all on here but this is just when backing out. Something totally different.

Or is this just 4HI vs 4LO??? Any ideas??