Ya' know, it's funny .... my two 2WD grass cutters will scoot up my hills without much of a whimper (when the transmission is cold) and I can pull a small trailer full of wood with the Craftsman. Virtually no traction issues at all -- no slippage or tire spinning as long as I'm not on mud. I added chains to help control downhill travel, but I'm not sure they do much uphill.
With the SCUT, I have to be in Low to climb a hill ... the same hill the little mower will pull a trailer of wood up.
Seems like it's more about machine weight and gearing/power to the ground than 2WD or 4WD.
I don't have any experience with ATV/UTV type machines though.