I know I had an old riding mower that had a 2 speed transmission in it, Hi/Lo F/N/R but i cant remember the manufacture.
there is one on EBay that may work for you it is item# 180607159985 or 180605979880 or 200530859863 or 350425293933
The last one has an open deferential that allows you to lock up one side. Magnatrac used them in their MT1800 to MT3000 Mini dozer's.
I have a MT1800 with this transmission and it operates like a skid-steer. It is possible to use this instead of the T20 but you will need to make an adapter for the input shaft.
The transaxle in my MT1800 works just like this