Hi Jason,
There’s a bit of discussion on this forum about the Northern Tools Ultra-Tow trailer. Some is under “Northern Tools,” and some is under “Ultra-Tow.” Here’s my two cents. I wanted to put the TD all the way back to the aft-most cross member of the trailer. However, that would have put the fenders in the way of the doors. There are two solutions: move the axle (and the wheels and fenders that go with it), or chop off the rails. I went the latter route. You can cut the aluminum rails easily with a reciprocating saw. My scrap pieces measure 19 5/8″, so I guess, with the kerf, I was going for cutting off 19 3/4″. The axle remains 100″ from the coupler. You don’t want it any shorter than 100″.
Good luck with your project.