Maxwell11, Interesting post:h
Only towing 6,500# with a 2005 CTD, I don't have an issue pulling my daughters GN Horse trailer at that weight nor does she!
Now lets talk our 5er, a 32' Copper Canyon at 12,360# GVWR, towed at 11,000+#. This with a 2001 2500 with 3.55's and maybe the difference a NV4500 5 speed manual. I do have a small chip for about 280 HP and 560# Tq.
The 3500 it is not stated if DRW or SRW should be within all numbers with a 12,000# 5er. I only exceed the GVWR of my TV which is only 8,800#, still well within all others.
I would think that 3.73's would do fine with that load.
Price on the 2003, maybe a tad high, but pre-emissions 5.9's are getting good prices, as they have a great track record.
Mine is currently at 287,000 miles and still pulling strong. Consumes just over a qt. of oil between 10,000 mile changes.