Well it took several months, but we finally sold the diesel, and found what we think was a pretty damn good deal on a "new" truck.
2007 F350, Former Shell Oil truck (so I'm thinking that it's had regular oil changes), high mileage, for an 2007, but well within our tolerance, and the price reflected it.
By my figuring, we should have 1500 - 2000lbs of payload remaining -after- attaching the trailer to the truck.
Black book for our area on the truck was $27K (factors mileage in). We got it for $17ish K.
The dealer delivered it to us today (from 500 kms (320 mi) away). Acid test tomorrow, when we hitch it to the trailer and see if it decides to keel over dead. :)