What are the options out there for TT auxiliary transmissions?
Layne Machine works makes two KC Warford auxiliary Transmissions that work on a TT. The Standard which is good if you happen to have a high speed rear end. Plus they now make a higher ratio for the TTs that have the low speed rear end. Visit their Website. Or you can look for a used Warford or Muncie. Expensive but you will be very happy with new from Layne.
There are many options, as Bob indicates. Anything from a Ruckstell alone to an auxiliary transmission mounted at the planetary output, another driveshaft unit mounted just ahead of the rear end, and a Ruckstell. You can gear things down to the extent it will "climb a tree" and go so slow you need to sink a post and come back a few hours later to measure progress. On the other hand, with high speed rear end gears and two over drives you can get it going much faster than I'd care to go in a 95 year old truck on 95 year old wood spokes. I won't even go into the brakes, but you get the idea.
What are you wanting to do?
Warford, Muncie, Jumbo, Lincoln .... there were probably others (?).
Universal (brand name) made both two and three speed models with and without accessory brakes built in, Rocky Mountain Six Speed were an under/direct/over, Moore were mostly used as underdrives but could be set up as an overdrive. All of these were offered in both car chassis and TT chassis versions mounted just ahead of the differential.
So many to choose from.
I have a 3 speed Universal on my TT wrecker, Warford with Ruckstel on my dump truck and Rocky Mt. with Ruckstel on my Express box truck. I like the Rocky the best. I have Rocky Mt, brakes on the wrecker and express. Need to get brake lining on AC brakes for dump truck. No way would I want to go back to stock in a truck.