D-mac,
Just be very careful of the actual weight of the Granby. My brother has a 2004 Granby on his '99 F250 long bed, and it is about the same loaded weight or slightly more than my Lance hard side 165-s. It just looks like a big white solid looking box HAS TO BE much heavier than a down position, soft-side. The biggest difference is tank size. He has much bigger tanks than I do and that adds measurably to the final weight. It is a myth that all soft-side campers are lighter than hard-sides. You just look at the specs, case-by-case, model by model and make your decision.
What ever you decide, you may have to start upgrading your suspension on your 2500 to carry even the modest weight of a small soft or hard side. I am looking for the holy grail for this and have gone through many permutations, which are on-going as we speak.
Do you have the upper overload (secondaries) on your 2500? If not, let the games begin.
Stable Loads? Lift springs? Anti-sway bar? E or F/G rated tires? Lots to consider here.
regards, as always, jefe