At a certain slope angle, almost no tractor will be able to do much ground engagement work going up the hill with an implement sized for flat land. Maybe a 4' land plane isn't much of a load to pull? I don't know, I've never used one? I can pull a pretty full 7' box blade up the 10% grade on part of my driveway but not with it taking a cut out at the same time. Normally I always try to use my hills to my advantage and do more work pulling downhill than up.
Also for a dry or loose gravel driveway, big softly inflated turf tires might work better than narrower r1's? Especially if the rears are loaded. In dirt or clay or mud, R1's perform the best