I used to teach auto trannies at the GM Training Center - here is the GM rule on towing automatics:
"35/35"
You can tow an automatic for no more than 35 miles at no more than 35 miles per hour.
An automatic transmission, unlike a manual, gets no splash oiling - all bearings and bushings are pressure fed from the engine-driven front pump. If the engine is not running, the tranny gets no lubrication. When towing, the tranny output shaft and rear drum turns from driveshaft input, and none of those rear bearings are getting any oil. If you exceed the 35/35 rule, you will destroy the bushings/bearings in the rear of the tranny. Anything less than 35/35 is okay and perfectly safe.
Lars