Probably best to lock out the O/D when towing, to keep it from shifting in and out of O/D every time you hit a grade. Flat lands it would be OK to run it. Since it appears to have a factory 7 way connector at the hitch, then a good trailer brake controller may be all you need to add. There are a number of good ones out there, I prefer the Tekonsha P3, however the P2 is similar in function with less features (such as the P3 has memory for settings for 4 trailers, the P2 does not) Also the P3 screen is annoying bright at night, even when switched to dimmest possible settings.)
The Tekonsha web site is all jacked up and refuses to show me the controllers, just the adapter cables. Here are Amazon links to everything.
Tekonsha 90195 P3 Electronic Brake Control , silver, Single
TEKONSHA 90885 / Tekonsha Prodigy P2 Electronic Brake Control f/1-4 Axle Trailers - Proportional
Tekonsha 90160 Primus IQ Electronic Brake Control
Tekonsha 3035-P Brake Control Wiring Adapter for Ford
Looks like the connector you would be plugging into will be an open connector under the dash to the right of the brake pedal.
This is the adapter cable with connectors on both ends.
Trailer Brake Controller Harness, Compatible with Ford, Land Rover, Lincoln, Mercury vehicles PART NO 3035-P You are looking for a plug dangling that the squarish brown plug on the left in the pic would plug into, It may be taped back into the harness.
You can also buy a single plug harness to fit the Ford that can be spliced to other brands of controllers (other brands of brake controllers are LAZY and don't make all of the custom cables that Tekonsha makes!
Do your due diligence on the 5.4L engine, as the early ones had some issues, I'm not sure if this is one of the ones the spark plugs blew out of or not (not enough threads) but if so there are kits to install inserts to "re-thread" the head. The biggest issue with any of the Ford engines is to use a torque wrench on the plugs and use the proper torque setting, a small amount of thread lube, and to make a point of removing and replacing the plugs on a "regular" basis so they don't stay in the heads so long that they seize up. The other big issue was the COP, or Coil on Plug. Each spark plug has a coil on top of it, and if you get a misfire and it turns out to be the COP, do yourself a favor and buy an entire set and replace them, as they will go out one after the other until you have replaced them all, one at a time, which is a hassle.
Charles