I've got the same van as the OP only it's a regular box and has the 4.10s and is a 2001. It's GVWR is 9500, GCWR 20,000 and max tow of 10,000 and it was purchased as a Cargo Van which I customized. I've now got just over 95K total miles with close to 60K of that towing with a GCW of between 16.5 and 17.5K. I get between 10 and 11.5 interstate towing and I run a mild 60HP tuner. It's a fantastic TV with plenty of power with the only limitation is with even a mild tuner you are EGT limited because of the lack of an intercooler and the crammed engine compartment. The 7.3L vans were rated at 20K with the 4.10 and 16K IIRC with the 3.55 which were the only two rears normally found.
Larry