i used to tow a construction trailer all over gods creation. the 2001 Dodge Ram i used to do this had three transmissions installed. the original, had that torn apart and rebuilt, then put a Jasper in it. it was a good truck and i didn't think it owed me anything, i put a lot of wear and tear on it and it did after all, make me a lot of money. if your going to be doing a lot of towing, the best thing you can do for your tranny is to make sure it is properly cooled, i would install a secondary transmission cooler.
transmissions today are quite a bit different than they used to be. most are either 6 or 8 geared now, depending on which truck you have.