The Ford Ranger we have in the UK is a re-badged Mazda 4X4 pick-up truck built in Korea or Thailand or Malaysia or someplace. I doubt that ANY parts (let alone engines) are compatible with Explorers. There are all sorts of issues that go beyond simply bolting a diesel to the Ex auto transmission. What would the 'computers' make of it for a start? Might be better to find a suitable engine and transmission from a suitable donor vehicle (Range Rover, Discovery, Land Cruser, Patrol, etc) and drop the whole lot in. I still don't know how you'd by pass the electronic brains without making your own wiring harness to suit the new engine.
The bottom has recently fell out of the UK Explorer market - you should be able to pick up a '98 model in good working order for around £1500 - £2000. Probably a much cheaper option than going diesel... Keep the car or transplant the engine...
Simon H