The DRW F350 had the 11k lb Dana 80 rear axle while the SRW's had the 9750 lb Visteon Sterling 10.5" axle. Things recently changed with F350 SRW getting a Dana 275 axle which has the same or better rating as the Visteon and the F350 DRW getting the Dana 300 which is an update of the Dana 80.
Remember that wheels consist of the rim and tire. We have not found the actual limits of the rims used on Ford's other than the printed specifications that align it with the tire rating. If you want to be safe, you should look at rim and tire upgrade.