the dana 300 is cast iron, gear driven, heavily supported for low range gears, shifter cables, clocking adapters, etc. its very compact with fixed output yokes front and rear that are cheaply upgraded to 1330 joints using JY parts. it uses twin shift rails that can function independantly, meaning one rail controls the front output and one controls the rear, H-N-L on each stick so you can choose from RWD High and low, FWD high and low, and 4wd high and low. stock low gear is 2.62:1
the np231 is aluminum cased, chain driven with a 2.72:1 planetary reduction section in the front. its either in high or low, and 4wd or rear wheel drive. no front drive option, no rear low option. it also incorporates the slip yoke into the tailhousing rather than driveshaft so theres a very long tailhousing on the back of the unit that just sucks. you will need to cough up $100 -$250 for a slip yoke eliminator which cuts this down, replaces the tailshaft housing and attaches a stubby fixed yoke. 231's arent bad units if its already in your truck, but id never suggest swapping to it. they do make a good crawl box in front of the Dana 300, but again, only if it bolts to your current transmission. the dana is what gives you all the options.
NWF makes a cable shifter for the dana 300 that will allow it to go just about anywhere. . someone on NEOW actually has one for sale.
problem with the OTT setup is you still have to adapt a minitruck gear driven crawl box to the back of your taco transmission, which requires a spendy adapter. so you'd be buying 2 tcase and 2 adapters.
you should be able to bolt a jeep AX15 5speed to the back of your 3.4 using an R150 bell housing from the 3.0 toyota with the jeep clutch and toy flywheel/pressureplate. that would allow you to run a box 4 rocks directly on the ax15. they're made by D.D. Machine and mate the np231 planetary box to the D300. if thats not deep enough, you can still go 4:1 in the D300 for ultra low.
if it were me, i would sell off the manual transmission, pedals, shifters, tcase, etc and acquire a toyota 3.4L automatic flexplate and torque convertor, R150 bellhousing, 92+ AW4 (23spline with a .500 stickout) transmission for $150 or less, control it without any computer at all using the rory desjardin rail shifter for $100 and then run the dana 300. auto wheels so smooth. theres no adapters and selling your current manual parts would probably leave you with money in your pocket.