2001 Durango R/T

So just got this vehicle about 2 months ago, and it’s been
great other then a few issues that occurred.. which need
to be resolved. Okay so 8 hour trip towing this Uhaul,
point A to point B ( 4 hours there and 4 back) everything
seems good, get there fine.. Then on the way back, going
up this hill (left overdrive on because thought it would
automatically downshift..) Snap! Sounds like the dang
transmission bands or engine blew lol. So pull off to the
side of the road turn it off, put it back in drive and it just
grinds a little and sounds like the driveshaft is just
spinning. So got the bright idea of putting it in 4wd and it
almost drives normal? But it’s only rear wheel drive ?
Anyone have this problem before? Hoping it’s not the
transmission, if anything the transfer case