An auto might be good for you since you are complaining about your knee.

Regarding going slow on the trails, it's easier to use an auto off-road than a manual. If you have a manual, depending on the difficulty of the trail it might necessitate other modifications such as lower gears in the diffs or a crawler box behind the transfer case. So the manual might require a few thousand dollars of other modifications to go as slow as you could go with an auto. So it's much more economical to get an auto anyway. You might add a transmission fluid cooler and then you're set with the auto. Compression braking when going down steep hills is still acceptable with the auto. The only disadvantage you'll see on a daily basis is the auto usually gets 1 mpg less.