Theoretically, if the front end was high enough so the rear bearing was bathed in oil, you wouldn't need to disconnect the driveshaft. From ztoolman44's response, it sounds like it doesn't get it high enough.
I am not doubting what ztoolman44 said but I gotta think that his output shaft was about to go anyway. I don't think a single 100 mile trip would hurt a t-case that was in good shape at the start of the trip. I once flat towed a 69 from northern CA to Austin TX (>1500 miles) without disconnecting (before I knew better!) and had no problems. But yes I agree with the advice - disconnect!