i had the 2 speed set up in rex x buggy and it didnt really matter about off road becuase they have a kind of clutch system so they can readily keep shifting from 1st to 2nd
it does get rid of 1 diff but youve still got 2 more to help you out
i used min off road all the time and iv got the monster truck version of the rex-x which i was using on wet tarmac with locked diffs and that didnt do any damage just kept changing down when it needed