[JRTJH]

If you go to the Ford Build Sheet, you will see that for the 2013 F250/F350 the rear spring packs are "rated" 6100/6730. This might be done with "mirrors and silk scarves" (magic) but I think it's more than a spring spacer to increase the spring rating that much.

Also, the rear axles are different between the F250, F350SRW and F350DRW' Here are the differences per the charts on page 83 of the build sheet:

Axle Shaft — Spline Minor Dia. (in.) 1.36 1.50 1.35 —
Spline Major Dia. (in.) 1.50 1.57 1.42 —
No. of Splines (Hub End) 35 37 35

As you can see, the axle shaft the spline diameter and the number of splines is larger on the F350 SRW. With dual rear wheels, the axle shafts and spline measurements return to "near" the F250 measurements. I don't believe the same "rear axle assembly" can be installed in both vehicles with these differences in axle shaft machining. I did not check the 2012 or previous model years, so they may have the same axle assemblies, but the 2013 F250 and F350SRW do not have the same rear axle or spring packs.