The 9" rear end was used in F100's, F150's, full size Broncos, Econoline E100's, E150's and most of Ford/Mercury/Lincoln's full size and mid size passenger car line during the 60's, 70's and early 80's. So that gives a good idea of what the axle's weight carrying capacity is.
All Ford 9" rears are not equal. There are small housing and big housing versions, as well as small bearing and big bearing versions and different axle shaft diameters and spline counts. So, they all have slightly different weight carrying capacities. 2500-4000 lbs seems to be the range of capacity that I have seen for the various versions of 9" rears.
Of course, you can have Currie, Dynatrac or Dutchmen custom build you a Ford 9" or Dana 60 rear to do just about anything you want it to do, if you have the money. :)