I am looking at buying a 2002 f350 PS and some of them that I have looked at have 3.73 rear end gears.
All the 2002 F-350 SRWs have a stock 3.73 ratio. On the DRW, 3.73 open axle was standard and 4.10 limited slip was optional.
I pull about 15-17K loaded with my 97PS and have no problem. What is the difference in pulling the same weights with the 2002PS with 3.73 gears?
Probably about the same. The '99-up has more power and torque with the charge air cooler (so-called intercooler).
Your GCWR is 20,000 pounds, so you'll be a few thousand pounds overloaded over the GCWR. And if your 2002 is a CrewCab 4x4,you'll also be overloaded over the GVWR. So if you must tow at night, then install air bags and use them so your headlights don't blind oncoming drivers. And if your truck is an automatic, don't leave home without installing a tranny cooler from a 6.0L PSD, and a tranny temp gauge with the sender in the pressure port on the driver's side of the tranny. NEVER
allow the tranny temp to go over 225°, or you'll soon be visiting John Wood.
If you don't like the lack of performance when towing, then first install a pre-turbo pyrometer, then a 60-tow or 40-tow tune from DP-Tuner or Power Hungry Performance. Even dragging just 8,000 pounds, I would not want to tow without my 60-tow tune. With you heavier load, you might prefer 40-tow. And if you never allow more than 1,250° EGT, the towing tune won't reduce the longevity of your 7.3L engine.