Well I picked up my 1999 F550 7.3PSD w/ hauler bed yesterday from CA and drove it to AZ.  When I looked in the truck there was a gear vendors box on the underside of the dash and another one mounted to the right of the steering wheel but on the bottom of the dash.  When I looked underneath the truck there was a gear vendors attachment between the tranny and drive shaft. 

Here is my question.  How do I know what I have?  The owner didn't even know what it was. 

Here is a little more info too.  We were doing 75mph on the way home and the RPM's were at 2650-2700.  I got 11+MPG doing 75MPH.  At 53MPH the RPM's were at 2000.   The truck comes stock with a 4.88 in it.  The tires are 225 R19.5's.  At first I was thinking it was just a torque converter from gear vendors but now I'm thinking it is just a normal gear vendors kit. 

I'm sure someone will be more familiar with these than me.

Later,

Chad