Put one of these on the truck:
CORRECTION: AutoPark 2000 (SmartPark is fixed mount for garages)...
It warns when you're getting close to something when backing up. Very simple to install: just tap ground and your backup lights at nearest taillight, and run a pair of wires to the cab for the warning speaker. It mounts to a bracket (supplied) that fastens on with the screws that hold your license plate in place.
I apologize in advance for the pics below. I took them at night and the camera doesn't focus right in low light and it wasn't important enough to do them right -- and I'm lazy, lol.
Anyway, the electronics are all in the piece that mounts on your license plate. Although they claim it's waterproof, I wasn't happy with the coating on the board so I took it completely apart, masked off the optics, and sprayed it heavily with urethane "conformal coat" for circuit boards. The lower part of my truck spends considerable time underwater, lol.
I put it on because the reduced visibility with the spare tire carrier have resulted in me backing into cars not once but TWICE. Now I get a warning at about 4 feet and a rapid "urgent" beep and change of pitch around 18 inches or so which with my tow hook and what not evaluated to about a foot left.
You can see where the speaker (really a stick-on tweeter) mounts. The beeps are loud but not obnoxious and they can be heard over a loud stereo. It's close enough to my ear that I can't miss it.
I bought it for $15 including shipping on eBay, so it's not expensive if you can find one cheap like that.
It senses ambient light also and extends it range 20% at night when visibility is even worse. You can adjust the range of warning somewhat using a trimpot on the board.
Overall rating: 9 out of 10. I'd give it more if it were better waterproofed as delivered.
Tons of them available on eBay for $25 including shipping. That may actually be what I paid, come to think of it. Still worth it and then some.