The capacities vary based on rear end gears and wheel size. The max weight is for the truck, the trailer and all the gear loaded in to them.
Assuming that it came with the factory tow package, the max weights for a 5.4L 4x4 are:
3.73 gears and 17" tires 7400 lbs trailer weight
Max combined weight (truck and trailer) 13000 lbs
3.73 gears and 16" tires 7900 lbs trailer weight
Max combined weight (truck and trailer) 13500 lbs
3.31 gears and 16" tires 6400 lbs trailer weight
Max combined weight (truck and trailer) 12000 lbs
Max tongue weight is 500 lbs
Ford also says to use a weight distributing hitch for trailers over 4000 lbs
The thing to remember is that cooling is usually the main limiting factor in towing. With 148k on the clock, your cooling system may not be as efficient as it once was. Towing heavy loads in hot weather will stress the system, so check it out, (hoses, flush and fill) and keep a close watch on the gauge.