Hi Mark,
Cool and unusual truck. I really like the Isuzu cab you've put on it. How do you plan to disconnect the rear steering for the highway? Another option may be to make the rear steering slower acting (less ratio than the front). It might mean a bit of tire scrub (= wear) but it may be worth the gain in stability. Another option would be to make the two front axles steer and have the two rear fixed. Obviously, a lot more work.
You mentioned that you're going to add a movable roof, any more details? Do you already have a plan? Will it be soft or hard sided? Will it be hinged or move straight up and down? Sorry for all the questions, just interested in how different people solve the same problem. Also, how did you manage to get such a large corporation to build the custom box, is it something they do as a part of their business?
Thanks