I have or hadmost of the variations of this engine. Sort answer is the '96 4.0l would drop right in. The engine from 96-98 is the same. There were some changes to the '99, I think cam profile. In '00 the head changed. I think there were some bearing changes around '00 to '02. I have an '03 and the mounting bosses are a bit different, but adaptable.
So, as I said, any eninge from 96 to 99 will bolt right in. An engine from 91 to 95 would work to. There is an extra coolant port in the head, and then EPA regs as far as earlier motors in later vehicles.
'00-up, the exhuast ports are a lot smaller. Still, the engine would bolt right in. I think 96-99 4.0L usualy bring $350 to $400 or so. Some of the latter ones bring a bit more.
Tom