I would kick myself for not doing the differential if I going to open for gearing.
That said, if you don't plan on crawling and getting a tire off ground, it's going to be of little use to you and a waste of cash.
There are other types of traction devices to consider... if I didn't have wheels leaving the ground on the regular basis, I would look into something like a mechanical limited slip. (Torsen style / Detroid trutrac)
- No maintenance when compared to a clutch based system
- It's always "on" and working (unlike a selectable locker)
- Works on and off/road
- Provides more traction and benefit for your use case.
- Seems like it would compliment ATRAC for the tougher situations where you might be doing some light crawling.