Diesel cool down during Idle unlike an unleaded engine that heats up. In low load ranges normally around 40% or less older diesels can wet stack. This is when the engine collects unburnt fuel at the injector tip and in the cylinder. Remember diesel operate off of high compression and heat. On newer fords, ford has helped with this problem. The trucks are designed to step up the idle at 45 degrees and 35 degrees. I want to say its something like 650 and 850, but dont quote me on that.
I guess look at what your hauling eric and determine if you feel a diesel dully would be worthwhile for you. If you start pulling heavy equip to do larger landscape jobs Id go for it. If not stay the way you are currently.
Why do you care anyway? You trade em off at the end of warranty anyway.