I second the NV4500 if you want a manual with an overdrive gear but it will be more than $2000.
You don't want to go through the installation swap without having a fresh tranny and transfer and as mentioned you need a newer (I think 1989+) if I remember correct round pattern NP205 and machine the tailstock on the tranny a bit or you need an advanced adapter which are $$$ .
New clutch.
Probably hydraulic master and slave.
Custom transfer case shifter linkage.
It creates a slightly longer drive line so you need to adjust length of both driveshafts or move the engine ahead about 1.875"
Also need a custom tranny mount made for the crossmember.
Probably some things I'm missing while I type this quick but you can easily see how this adds up to quite a bit more that $2000.
Guess how I know. You can take a look at my build thread in my signature.
I can't comment on a tranny change to auto with overdrive.
It is impossible to maintain the same bottom end pull and lower highway speed rpm without a tranny change, however, in all honesty only a gearing change is probably the easiest route if budget is any concern.
I'm not sure what gear ratios you can get for your axles, however, I think you would be looking at either 3.73 or 3.55 considering your set up without any other changes. Kind of a middle ground gear with 35" rubber and your drive line. At 65 mph it would be around either 2460 rpm or 2340 rpm. So not a huge difference. I guess it depends how torquey that engine is on the bottom end which one would be better.
I love the site linked below. Use the drop downs to enter your tranny, transfer, gears, tire size and then you can play with the outputs and compare side to side.
http://www.grimmjeeper.com/gears.html