sounds like an ICP fooler to me.. a "poor man's" tuner for the 7.3 was to fool the ICP into running higher HPOP pressure.. (the 7.3 is an excellent motor.. however you will want ot learn as much as you can about its electronics and the HEUI system.. it will save you headaches and $$ in the future if it ever acts up)..
anyway.. im only guessing on the "device" under the steering wheel.. there were many sold.. they effectively change the resistance of the sensor when you turn the knob.. they dont often cause stalling..(they can turn on the check engine light) but they can definitely increase the "growling" of the motor and it may increase power,although there are decidedly better ways to "soup up" a 7.3 then that..
you can play with it as you drive..
looks like a great van! im a fan of the 7.3 Diesel. (and its Navistar counterpart the 444E)
-Christopher