As above.... Be Careful.
I use my BX to move my boat all the time. It can be up around 5000 lb when there is fuel, water, and the rest of the junk in it. (anchor, vests, water toys, so on) I only move it in low range, and I make sure it's in 4wd and the FEL is on. You need to use 4wd since you brake is in the trans, and not on the wheels. In 2wd, it's too easy to slip a tire, and have it feel like there's no brakes at all.
With that said... only try to move a heavy trailer on flat ground !!!! It can get ugly FAST if you are trying to move it on a hill.
Oh... on last thing... since it's a big trailer, and I will assume you have electric brakes on it... make sure you put the trailer brakes on a long pull cord so you can yank the emergency switch out if need be.
https://goo.gl/photos/YRtFaiUun4KLUtrK7