I believe you are confusing the spring rating with the rating of the axle. The link you provide lists only two AXLE ratings,
F250 - 6200lbs
F350 - 7200lbs
While the numbers you quote are what I see listed for the spring packs.
So no matter what spring package gets selected on an F250, it's rear axle isn't as strong as the F350's with the same spring rating
The link I provided gives the F350 SRW trucks RAWR as 6290 or 6730 or 7000 lb depending on spring pack selections which is what we find on Fords door placard.
As Bedlam says the F250/F350 SRW uses the same Sterling rear axle. The different spring packs/tires and wheel ratings determines the different RAWR numbers.
Where are you coming up with 6200 and 7200 numbers as a RAWR.
I'm looking at 3/4 ton gas trucks right now. Every F250 6.2 4x4 crew cab 18" wheels and tires I've found has a 6100 RAWR on the door tag.
