If you have 4.10 gears, your GCWR is 18,500 pounds. If you have 3.73 gears its only 15,000. Keep in mind GCWR is just performance and warranty rating. If you're happy with it, great!
FWIW my typical combined weight is 22,000 pounds and it does very well at that weight (22k is the GCWR for the E450-V10 with 4.56 gears).
PS - The GCWR is in your owner's manual. Its a chart which lists the GCWR for each engine and differential ratio combination.
2000 Ford E450 V10 VAN! 450,000+ miles
2014 ORV really big trailer
2015 Ford Focus ST