Auto 4wd

[MarkD]

Can someone please explain this feature to me?

 

 

I also get a "groaning" sound at full steering lock with the transfer case in this mode.

[Shaners]

When You select the "Auto-Trac" mode on your truck, it locks the front differential (hence the groaning noise), but it still free-wheels at the transfer case.  When rear wheel slip is detected, the transfer case "locks" and traction is applied to the front wheels (as well as the rears).