I don't know why they haven't done it in the recent past, but you wouldn't want one now- the stupid EPA's unrealistic emmisions requirements would make it a POS bigtime. Our big trucks at work have $7,000. worth of emmisions on them, SEVEN THOUSAND DOLLARS above and beyond the price of the truck...
The trucks are being called "air scrubbers" because the EPA wants the particulates that come from the stack to be CLEANER than what went into the air filter.
My '99 model I had got 7 MPG working hard or 8 if you took it easy pulling 80,000 lbs. The '04 I have now gets 6.5 pulling an empty because of the EGR they put on them that year. The truck has 730,000 miles, and the only problems I have had out of the engine? Five (count 'em, FIVE) EGR valves and an EGR cooler. The valves cost $500 apeice without labor to replace. I have no idea what the cooler cost but it wasn't free.
The new ones with the 7K of emmisions stuff has a catalytic converter with a soot incinerator. Uses fuel to burn the soot out every so often. The first truck to get 15,000 miles on it used 92 gallons of fuel in the incinerator- 92 gallons of fuel that didn't propel the truck one damn inch. With all that crap you KNOW that check engine light is going to be on all the damn time. They preach MPG and no idle time, then pull this crap. It takes 45 minutes at idle to complete a burn cycle!
Yeah, come on, EPA- I'm bent over, come dry hump me some more... %&$*#@!! liberals...