December 2008 HOT ROD had the article.
What I was interested in were the "auxiliary 2-speed transmissions".
Gear Vendors under/overdrive transmissions the most awarded auxiliary transmissions. and
U.S. Gear Corporation Home Page are the two sources listed.
On HOT ROD's Ford Ranchero,and at 65-mph cruise,they picked up 2.5 mpg with the Gear Vendors unit behind a C4 trans.
The units allow gear splits between any gear.They ended up with a 6-speed.I would get a 10-speed if installed in the Toyota or 8-speed in the Dodge.
Price isn't mentioned anywhere in the article.If they are $3,000 that's really pricey as that would buy two complete transmissions for a Chrysler minivan.
The only way I could handle the expense would be to retain the original driveshaft and de-modify the truck at sale,transferring the OD unit into the replacement truck.
Nothing easy about saving energy!