Just to be clear; this is not mine.

'Worked when pulled. It came on my truck. And i didnt like it. It is for a dodge 3500 with a 5 speed nv4500. It is for a np241 transfercase. I have all the parts that make it work. The switch and relay box. And the wiring. My truck is a 1998. So im sure it works on 98-02 trucks. $400 obo

www.facebook.com/marketplace/item/869292778279461

Interesting piece. But how much OD do you need on a factory truck? Or are yoiu splitting gears?

On a level highway
with little head or side wind
an unloaded pickup truck drivetrain
needs more OD
to get anywhere close to max fuel economy or minimum engine wear.

I am tempted to say
“up to 2% grade hills”
instead of “level.”

The countershaft bearing is already failure prone in NV4500 trans. You add another overdrive and spin it faster is just asking for trouble.

That's not how an add on overdrive works.