(sorry if this is a second copy - the first didn't seem to come through; log-on problems)
First, you have my sincere gratitude/respect for (i) attempting to keep up the dream, and (ii) attempting to keep up your wife's spirits.
Second, although i don't have direct experience, as the sister in law of a woman who has MS, I'd say the following:
(i) forget about plans for "a few years from now". Go now or as soon as possible - MS is very unpredictable, and what may be a healthy year now may not last. Take advantage of the healthy years to get out on the road
(ii) modify your plans to allow for an option that gives your wife the chance to go without relying on her own muscular strength. Either 4x4 or pillion/sidecar may work - just something that gives her the chance to enjoy the trip without stressing about the physical requirements it entails. MS is very up-and-down; some days are very good, and others are very bad. You don't want your wife abandoning the trip because of stresses about the "bad" days - what you want is to keep moving!
(iii) get in to a specialist early. There are some amazing new therapies that have great potential, but that need to be started right away. Keep in touch with that specialist as you're travelling
(iv) the psychology of your wife is vitally important both to the success of the trip, and her future health. There are some really incredible success stories (my sister in law was supposed to have been bed-ridden 2 years ago, but is still up and healthy through a combination of drugs, healthy psychosis and a very aggressive approach to exercise), but they take work. Getting on the road could be a *great* therapy as long as combined with sound medical advice!
All our best wishes,