The Transit is a super nice van, with lots of power, but talking with an aftermarket up fitter, it's just not built for towing. Problem 1 is the guibo rubber driveline joint. Towing heavy trashes this joint. This particular up fitter replaces that joist with a proper u-joint. The other probablem is brakes. The Transit brakes and tires are so small it cannot stop any more weight (apparently stopping in case of trailer brake failure is part of Fords GCWR criteria).
Even though many users are pulling way overweight with their transits, the risk of trailer brake failure is a real one, and I decided to stick with Econolines for this reason. I've had my brakes fail several times. Twice because the connector came unplugged, but I've also had crummy trailer wiring too.
My E350 has served me great, with no power train failures so far at almost 430,000 miles. I've decided my replacement will be a 2018 econoline, either with an aftermarket van body (SportsMobile or TurtleTop) or I'll graft a wrecking yard van body onto the new cutaway cab.