The D-30 will probably be ok, if you truss it and get quality shafts, the AX-5 will not handle the torque at all, most 4BT engines have a trans not too far away, its better to use the trans the engine had to start with and modify the shifter and mounts so you will save a fortune on trying to get just the right bell housing, flex-plate, clutch, etc etc. with that much torque your going to want a little bit more stout transfer case too, the NP231 would probably do ok for a long time and its easy to rebuild but getting the shafts mated would be more expensive than finding a transfer case that will mate to the trans on the 4BT.
If the Engine has turbo or if your thinking of adding turbo then your going to need an inter-cooler, so make sure you mount the engine far enough back to accomodate that, and the radiator from the donor vehicle is the best to use if your not going to buy a high capacity aftermarket radiator, they get pricey fast.
This is an awesome swap when done right, Ive seen a couple 4BT equipped Jeeps and a few less obvious things like go carts and franken-4-wheelers, but it is a very complex swap to do well and expensive to do right, thats you dont see many of them. As far as street legal goes it depends on where you live, here in Wyoming you are required to liscense 4-wheelers for street use, anything with lights and an engine capable of 35 MPH or more is street legal here, places like Az, Co, Ca, Wa, Or, and a few eastern states have a peculiar set of laws, the engine must be newer than the vehicle its in, some states, like California, it must be an engine that was originaly available from the manufacturer, all emissions equipment must be not only present, but working, all OBDII computers must be programmed for that vehicle with proper VIN number and specs, and the list goes on and on, check your local laws, or, move to Wyoming lmao, here you can quite seriously liscence a 454 equipped go-cart (and I dont mean sand-rail), here it has to have at least one headlight and one tail light/brake light, and if it doesnt have turn signals the drivers arms cant be obscured from view (thats it), we dont do emissions, the wind blows all our smog in to Nebraska.
4BT Jeeps have massive torque, they are monsters when it comes to crawling, you have to decide if its worth it and if you can afford it, then check the laws in your state, if your state requires emissions checks or inspections anyone qualified to do those checks can tell you what the laws are. Good luck and for what its worth I really hope you decide to do this, the more 4BT Jeeps there are the better.
On a side note, in europe and maybe Canada, Europe for sure, the TJ was available with a factory diesel engine.
