The F450 and F550 can be 2wd or 4wd - This is the same for the Ram equivalents.
The F350 uses an 11,000 lb rated Dana 80 axle. Ram and GM have AAM's axle which is rated about 500 lbs higher in their 3500's. It will not immediately fail if you load it it heavier, but service life will exponentially decrease as you go beyond the rating.
FYI: With a 1000 lb tongue weight on a 42" SuperTruss plus 6000 lbs of camper, my front end unloads about 300 lbs even with a WDH equalizing the tongue weight. With just the camper, I have less than 100 lbs unloading off the front.