I understand why you would think that going off published specs but you are incorrect. The rear axle on the 250 is IDENTICAL to the SRW 350's with the same part numbers. It is de-rated from the factory just like the GVWR of the truck is to fill a different market segment of a Class 2 truck.
The XLT Package that the OP has comes with 18" rims which are also the same as a SRW 350 with the same tires rated the same.
So, like I said, it's the same truck.
F250 Left Rear Axle BC3Z4234C
F250 Right Rear Axle BC3Z4234D
F350 Left Rear Axle BC3Z4234C
F350 Right Rear Axle BC3Z4234D
Here is a link to the question being asked directly to a Ford engineer when they were on the FTE forums answering questions about the 2015 motor update. They were only there to answer questions about 2015 so the MY is mentioned but is irrelevant as they are all the same.
Ford Engineer PostI had links to an online Ford dealer to verify the axle part numbers but they updated their site and the links are dead. I'm too lazy to look them up again but if you want to do your own homework, look them up and you will see the part numbers I posted above are correct for both vehicles.