[B. W.]

Here's how I see it:
change ring & pinion ( on the cheap, just do the rear like you said)

gear vendors - but you can only run this in 2wd, must shorten rear driveshaft

ranger overdrive by advance adapters - (they call it a gear splitter because you can shift into OD in any gear) cheaper than gear vendor, fits between trans & bell housing, can use in 4wd, must shorten rear, lengthen front drive shaft

NV4500 trans - gives you a 5th gear overdrive, cheaper than gear vendor. Depending on how you do it, close to or a little more than the Ranger

the quickest, cheapest, easiest option is to change the rear gears only