Go over to a Ford PSD truck forum and look at the sections that cover the OBS trucks ('94.5-'97). There, you'll find a plethora of people that are pulling the weight in question with those trucks. Most always, these people have invested a little bit of time/money to make the trucks do this effectively. This involves upgrading the transmission, engine injectors, intake, exhaust, intercooler, etc. Rarely do you see them changing the axles or suspension, with the minor exception of adding air bags. The 7.3L PSD's of this era had a typical tow rating of 12,500#'s with the 4:10 gearing. However, folks are showing that the truck is capable of far more than that. With a little patience, you can find a decent dually crew cab with 250K to 300K miles for $5K to $7K. You can do all the performance enhancements listed above for about $5K, but more than likely, you'll find that the previous owner has already done some of this.
As a disclaimer, you might also find the same thing on Chevy and/or Dodge forums, but since my big truck is a Ford, I don't spend time looking at the other guys.
NOTE: DO NOT buy a truck of this era with over 300K miles unless you know the owner or the truck has a very detailed history report. Once the odometers reach 399,999 they roll back to 300,000. If you buy one with over 300K miles, it could very well have 400K, 500K...not that these trucks can't handle that of course.
With the way things are going regarding tow ratings, if you wait a few years you might find that the Ford ecoBoost will be rated to haul that much...and have no doubt that some on this forum will tell you all about it :W
