The 2001 4.0l will be the SOHC 4.0l so will need new timing chains as well as head gaskets so pull the engine and trans to do the work.
Also replace Knock sensor and thermostat housing, both are know issues with 4.0l SOHC engine, like the timing chains.
You need the trans with the motor because 3.0l trans bolt pattern won't match 4.0l bolt pattern
Motor and transmission mounts from the 2001 will bolt into the 2000 frame.
You will need to swap engine wiring harness and computers of course.
And Ignition key cylinder and door key cylinders, because of the PATS anti-theft system, PATS uses the Key ID to allow starting so hopefully you got at least 2 PATS keys with the 2001
You don't have to change the door cylinders but it is nice to have one key for unlocking and starting.
Drive shaft should swap over if both are extended cabs
Go here to see what axles are in each, using drivers door label: http://www.therangerstation.com/tech...le_codes.shtml
This will help you to decide which axle to use
OPEN means only 1 wheel gets power, the easiest wheel to turn
L/S means Limited Slip, both wheels get power so better than OPEN, Chevy calls it Positraction, Ford calls it Traction-Lok
3.73 is most common
3.45 is better for highway driving, lower RPM at 70mph so better MPG, but not as good for towing or heavy bed loads
4.10 is better for towing and heavy loads, also larger tires