If the rear axles are the same 10.5" for both 250 and SRW350 what gives the 350 the extra 900lbs rear axle rating?
The rear axles are not the same. Those who claim that all Sterling 10.5" rear axles have the same 9k weight capacity are uninformed.
Ford owns and runs the Sterling axle plant in Sterling, Michigan. The engineers at Ford and Sterling Axle work together to produce the axles with the specs that the Ford designers need. Although the housing and the 10.5" ring gear is the same, there are numerous other parts that determine the actual weight limits of the axle assembly. Axle shafts, bearings, and various gears, for example.
Over the years, the Sterling axle factory built the Sterling 10.5" axle assemblies with weight ratings from 6,000 to 9,000 pounds. Ford specified the 6,084 weight capacity for the '99-'04 F-250 and 6,840 weight capacity for the'99 '04 F-350 SRW. Those weight capacities matched the weight capacities of the tires available at the time for those pickups.
That's no big surprise. If you study cost effectiveness, the higher weight capacity costs more money to produce than the lesser weight capacity. So wasting money by specifying the same axle weight capacity for the F-250 as the F-350 SRW would get the engineer fired by any decent MBA manager. Same for springs and other parts of the suspension. Remember that Ford produced millions of trucks with 10.5" Sterling rear axles, so a few bucks difference in cost per axle adds up in a hurry. Engineers hate us MBAs that hold their feet to the fire on cost per unit, but that's what it takes to make a profit.
Beginning with 2005 model year, Ford increased the GVWR and GAWRs of the SuperDuty SRW pickups. To meet the higher weight specs, the Engineers at Sterling Axle had to redesign the SRW axles to meet the new specs. F-250 rear axle weight capacity was increased from 6,084 to 6,200 pounds and the F-350 SRW rear axle weight capacity was increased from 6,840 to 7,280.
Ford did build one Sterling 10.5" rear axle with weight rating of 9,000 pounds @ground. The 2005-'10 F-350 DRW rear axle was rated at 9,000 pounds for SuperDuty pickups with a 5.4L gas engine, so perhaps that's what confuses the folks that claim all Sterling rear axles were rated at 9,000 pounds. They probably read something on the Sterling website that said the 10.5" axles had weight capacity UP TO 9,000 pounds. (Rear axles behind the more powerful diesel and V-10 engines in duallies were Dana 80 axles rated for 11,000 pounds, not the Sterling 10.5" dually axle behind the 5.4L gas engines.)