thanks for the super fast replies, I posted this but havent had time to check it until after the holiday weekend.
Yes changing the differentials are not an option because there are 2 of them and I really wouldnt mind it on the straight rear axle, but the front diff with its offset IFS system (nissan being one of the first to offer it) would be a major undertaking. Not undoable, but maybe beyond the point of diminishing returns. I can find no vendor that makes aftermarket ring or pinion gearsets for the nissan 720 pickups, and most of those aftermarket sets for toyo are to lower the overall ratio for off road, Ive never seen one to raise the final drive.

The tire size are 31 x 10.5 x 15 which are right on the ledge where any larger and the tire vendors start to suffer from insanity when they price them, and it would Drastically alter the road performance, plus Im already rubbing a little on the front tires, I really dont want to increase tire size, that would be about a thousand dollar fix.

Yes this model did have a divorced t case and trans. I did design a 2 speed gearbox to be installed between the t case and the trans, using a set of planetary gears, but the simple machine tools I have were not precision enough to make it work totally balanced.

Even the most modestly priced aftermarket OD gearboxes are waaaay outside the reasonable price range, but that would be the easiest answer to the problem. I even looked for a jeep t case with the little "kit" that changes the input shaft to a set of planetaries. So since I might have to replace the tranny anyway. I figured that I might run it by you guys and see what my fit options are.

I guess my first choice would be to find a mb trans that would bolt up directly to the bellhousing from another model that, while still being a 4spd, would have a significantly taller final gear. For instance the trans I have in it now is a 4spd with 1-1 being its final. If I could find an MB 4spd with 4th being .85 - 1 then it would accomplish the same thing. While there are 4spd trannys all over the place they still have overdrive, they just use 4th as the od instead of the manual trans using 5th. THat would be my first choice and the easiest fix, I would think they would have such a transmission in some of the gas sedans, and if I could find out which transmissions would bolt up to my 617 turbodiesel, I could search gear ratios until I found one closer to my liking.

Short of that, I guess my second preferance would be to come up with an adapter plate to adapt to another relatively modern 4spd auto with OD and a lockup torque convertor, might as well get all of the advantages.
Its just that I went through a time that I was ready and willing to fab or adapt anything to anything, hence the MB diesel in a Nissan frame with a Mazda body - basically all the leftovers, but now, Im trying to avoid the heavy fab work for a while.
The RPMs I am looking for are right in the sweet spot 2500 to 2800, so if I can get 75mph at those rpms ill be happy. I think redline on this engine is just over 5k, but I dont want to get anywhere near that for a sustained period of time.
The nature of the diesel dictates it needs to be run at or around that sweet spot to get max efficiency , power, longevity ect. Thanks for the replies and the ideas, I could not have even built the Niesel without ALOT of help from this forum.