Towing 17-18klbs with a 10+ year old dually won't be the same experience, but with an added exh brake or 6.7 deleted with the turbo brake, I'd do it. I wouldn't like towing that much with a 48re trans though. Either G56 behind a 5.9 or 68rfe behind a 6.7.
You appear to know your loan terms and equity number. Remember, you've paid off 2/3 of the interest already and less than 1/2 the principle though. However your payoff is more than the cost of an older truck still, so it may still make sense financially even though the bank already got most of their cut on you loan.