Ford

Is it the timing chain

I don't remember a 4.0 v8 in the explorer ever. Whatever engine you have cranking fast like it has no compression does sound like a timing chain issue.

Right, timing chain in that one, tensioners , chains, did you hear any abnormal noise coming from the front of the engine? Like Guy said, check compression, may be a timing issue.

4.0 v6 of that year is a SOHC engine, cam on each side with tensioners, guides, etc. The front one is a real pain to change. If the rear one is the problem the engine needs to come out of the vehicle. If the rear guides/tensioners, etc are the problem, and the engine has to come out of the car, then it is worth while to just purchase a remanufactured (not rebuilt) engine and drop it in.