Using the TOW / HAUL option will disable the MDS, I think it also locks out 8th gear ( with the old 6 speeds it locked out 6th).
Also, with the old transmission at least .... if utilized the manual shifting buttons on the steering wheel and selected the high gear, that would also disable MDS. Once you place the trans in park it would default back to "auto" mode which would enable MDS.
I don't have my truck yet to confirm (sitting at the dealer) but maybe someone who's pimping around in theirs can try both and report back.