Depending on where you are and what your budget is you might want to check out Utility Truck and Trailer.
The one in Boise does outstanding work.
Our last truck was an old city work truck that I got at auction. It had a flatbed with a liftgate. I went to Utility b/c they did the original conversion. From what the guys there told me more modern liftgates are designed to be able to pull a trailer. With quite a bit of welding labor and fabrication work it only cost me $1000 to have the liftgate modified to pull a trailer and some other work too; which was more than $250 cheaper than only other three fab shops in town quoted me.