try all these, in no particular order:

http://www.gearvendors.com/
http://www.usgear.com/
http://www.advanceadapters.com/acrobat/saturn.pdf
http://www.high-impact.net/transmission_and_gear/rangeroverdrive.htm

What truck is it in, and what are your goals?
there are many overdrive transmissions that can be swapped for the same and mostly less money that the above "add on" overdrives.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-