90 and earlier won't work without a significant rebuild and parts swapping.
91-97 won't work without a fairly significant rebuild and parts swapping, less than 90- but not easy.
98-01 are bolt in.
More detailed info:
http://www.naxja.org/forum/showthread.php?t=1053970
Make sure you get one that's the proper 2wd vs 4wd version. Converting back and forth from 2wd to 4wd is as difficult as converting from a 90 and earlier to fit a 98 and later, not for newbies, unless you really want to do a rebuild that is more complex than simply rebuilding your existing transmission.
You can make a 91-97 trans (slight wiring differences across those years, but nothing complex) work with a 98 and later if you pull a 97 (only 97, make sure the part number is 5602 7951) TCU and install it and do some wire splicing.