Buy a SRW truck. It's not just the axles, if you have a bed, it is also different with much larger wheel openings. the swap can be done, but if you want a smaller truck, sell it and pick up a W250. If you get a Cummins W250, it will still have the same axles, just SRW. Or you might get lucky and find someone with a W250 wanting to go dually, but it may take you a while to locate such a person and reach an agreement.
1988 Dodge AW450 Ramcharger-318TBI-NP435-NP205-D60s
I put on my robe and wizard hat. - Bloodninja