Has anyone added an OD aux trans to their truck?

[5Tractorguy]

Was thinking of doing this in my '77 K25 so I'd have a "highway gear", or at least drop my RPM from 2800 to around 2000 at 65mph. It would have to go behind the t-case and could only be used in 2WD, which isn't a problem for me. Preferably a 2-speed box, direct and OD. Would rather not go with the Gear Vendors overdrive unit since I don't have an extra 3-4 grand to play with.

Anybody successfully done this with pictures? Or, any recommendations on a truck salvage yard/company that may have used auxiliary transmissions?

Any direction/thoughts are appreciated.