Kinda....
The 9900 lb axle has thicker axle shafts and different bearings. However, your point it valid that the axle assembly itself is NOT the weak link here. The tires are the limiting factor as well as spring sag. The sag can be corrected with air bags which basically leaves us with the tires as being the item to focus on.

Stick with E rated tires with 123 load index inflated to 80 psi and no more than 6830 lbs on the rear axle and the OP will be fine (provided airbags are installed).

 

My experience owning and towing with a 7.3 CC 4x4 SB for the past 14 years...

1. You MUST stay within your max tire ratings at max psi per the tire manufacturer.
2. If your F-250 came with the auxiliary helper spring, then your realistic RAWR is 6830 lbs provided your tires are rated for 3415 lbs at 80 psi. For your year, the F-250 with auxiliary helper spring, 4" block and a rating of 6084 lbs is exactly the same hardware as the F-350 SRW models which have a RAWR of 6830 lbs. Just an FYI...
3. Airbags are recommended, then the auxiliary helper spring become irrevelant.
4. Keep your GCVW under 21k lbs unless you plan on upgrading your transmission. If you have already upgraded to a HD4R100 or better, then get yourself a 6.0 trans cooler and you should be good.
5. E rated tires with a load index of 123 (single)minimum.

In the nice to have but not absolutely necessary category...

1. Hellwig rear sway bar
2. exhaust brake (either aftermarket or wire the OEM unit)
3. Fuel pressure gauge
4. Load index 126 tires (3750 lb rating @ 80 psi) LT285/75R16

A stock 7.3 with 3.73 gears and stock transmission will tow a GCVW up to 20k lbs all day long provided you upgrade the transmission cooler. Pretty much anything under 21k lbs is easy peazy even in mountain conditions.

At 23k-26k lbs gross its best to have some performance upgrades such as stronger transmission and PCM calibrations in the 40-60 hp range. If you use a quality tuner, then you can run a 40 hp calibration while towing without any additional supporting mods. 60 hp or greater while towing should have an EGT gauge in case you need to regulate temps with the throttle. While towing in this weight range, an exhaust brake is almost a must because you would have exceeded your braking capacity at highway speeds.

Anything in the 26k-30k range is going to require a 4.30 gear or lower (numerically) since you will now have exceeded the capacity of your cooling system. At this point you really need a F-450/550 anyway since their brakes are up to the task.

According to the scales, the weight of my fifth wheel is 12,600 lbs empty. With stuff, it can weigh anywhere around 15k-17k just depending on what I have loaded in there at the time. Last time I scaled it while towing with the F-250, it had 2680 lbs of pin weight. Last time I scaled it while towing with my F-450 the pin weight was 4120 lbs because I didn't have anything in the garage. So, the pin weight can vary depending on how you load the trailer...especially if you are looking at a toy hauler model.

When towing with my F-250, the weight on the rear axle was 5680 lbs and GCW was almost 25,000 lbs fully loaded with fuel, supplies etc (measured at a certified scale). Towing that same trailer with my F-450 put my GCW around 28k lbs gross. Engine had enough power but I did have to watch my coolant temps on the long hard climbs. Performance wise, the F-250 matches or outperforms the F-450 in all areas except for braking. Both trucks have heavily modified transmissions and custom PCM calibrations. I had no stability issues with the SRW truck after I installed the Hellwig sway bar, that thing made a huge difference.

I think the trailer you have picked out is fine provided you verify the weights you posted. I would recommend air bags, stronger tires and sensible driving. Don't haul around at 75-80 mph and you should be fine. Slow down and don't be in hurry....so you get there 15 minutes later, big deal. At least you and your family get there and can begin enjoying your trailer.

Oh yeah....one more thing. If the trailer has 15" tires then negotiate for 16" LT tires. If 16" tires don't fit then find a trailer that will accept the better tire. ST trailer tires are complete junk IMO....I refuse to run them on any of my trailers. Since making the change to 16" LT tires, I have not had any blow outs (knock on wood).....been about 11 years now.