It's a rule of thumb to always drop the driveshaft when towing a bus/semi/straight truck etc. The bushings and gears inside won't be getting lube if the input shaft in the trans isn't turning. If there is no lube, then stuff can be trashed fairly quickly. Allison says to never do it, and that doing so can cause extensive damage in just a mile.
This also applies to manual transmissions as well.