David Stacy from Trucks used Gear Vendors over drive unit behind his project truck, here are the 2 web sights that can help. The first is for Gear Vendors and the seckond is for the project truck from Trucks TV
( Gear Vendors under/overdrive, Frequently asked questions. ) (Official site of Stacey David's Gearz | home
2008_0_0&content=projects&em2463=&ek=96289 )