I think you might be confusing GVWR and GCWR. GVWR is the total allowable weight on the axles. GCWR is GVWR plus the trailer.
The GVWR of that truck is 8,800 lbs but the GCWR is 15,000. Max trailer is 8,500 with the 3.73 or 11,500 with 4.10s.