Hi stillworkin, I have worked on these trucks for many years, you will need to remove the mast to remove the transmission, however the mast removal is not so bad. You said the oil in the transmission was not burnt so I would suggest you look at the dump valve on the transmission control cover first, this has a small cup seal in it that sometimes swells and causes the valve to stick and can give the problem you have. It is easy to change the seal so worth checking this first.
Hope this helps.