Mechanically, all of the 242ci engines are close to the same (there are some differences - the cylinder head was revised a few times, and the main cap brace was added for 1996.)
The first generation, as mentioned, is "RENIX" - and that's what you've got. The system was spec'd by Renault and built by Bendix/King. It's actually quite robust, adaptable - and best of all - pre-OBD. I've never been a huge fan of OBD-I (principally because there was never a real standard for it. OBD-II makes more sense.)
RENIX is actually fairly easy to troubleshoot as well - all you need is decent skill with a multimeter and a useful manual. You can find the RENIX FI manual in the Tech section at Pirates of the Rubicon, and I've kicked open a head-shed for RENIX-specific discussion (groups.yahoo.com/group/RENIXPower.) I think the RENIX FI manual got upped there as well, I don't recall (and I know I didn't put it up there.) Just make sure you say something more than "I want to join up" if you decide to sign up there - or at least let us know it's you (we've been fighting SPAMbots of late.)
The RENIX system was used on both the AMC 150ci/2.46L I4 and on the 242ci/4.0L I6. There is a good deal of parts commonality as well, save the ECU. Most of the electronics and sensors are GM/Delco units, which saves you trouble in finding them and saves you a few bucks on most of them should you need replacements.
Your 1990, if an automatic, will have the AW4 (a Toyota design) four-speed automatic and 3.55 gearing, which complements the transmission ratios well. If it's the five-speed manual, you'll have the AX-15 with 3.07 gearing - the AX-15 is a decent box (way much better than that wretched Peugeot - I went through four of them in my 88!) but the 3.07 gearing is all wrong. If you stop using fifth gear and cruise around 2500-2800rpm, you'll note an increase in fuel economy.
Which transfer case? If you have a "4HI Full-Time" option, you have the NP242. If not, you have the NP231 (and you'll have a vacuum disconnect Dana 30 up front. The rear will be a Dana 35 - the C8.25" didn't start showing up until 1991, and the D44 quit being installed in early 1988.) The NP242 was found in the Limited edition and some high-end Laredos (the ones with "Power Everything" - sort of an "economy Limited.") To my recall, the NP242 did not come from the factory behind the manual transmission, just the automatic.
Your alternator is a Delco CS-130. Therefore, the Durango/ZJ/WJ upgrade will not work for you - but that's really OK, because you can score upwound Delco units fairly cheaply anyhow (and I prefer the Delco to the ND. The Delco has been more reliable, in my experience.)
Monitor the condition of the ground strap between the cylinder head and the firewall - not for nothing is it called "The RENIX Killer." It provides the main ground path for the chassis (to the battery,) and therefore continues the ground plane between the sensors and the ECU - which is mounted just above and forward of your right knee as you sit in the driver's seat. Adding a secondary chassis ground from the battery negative terminal to the fender liner will now go amiss, and often solves quite a few headaches before they start. Use min. 4AWG wire for that one (I know, the OEM mains are 6AWG. You'll want to replace them sooner or later anyhow. I can help you with that - hit me backchannel...)
All in all, I prefer the RENIX over any OBD, and OBD-II (1996-2001) over OBD-I. Then again, I cut my teeth on 50's and 60's Corvettes & 60's and 70's Camaros, so I'm used to doing my own thinking.
You can find FSMs on eBay with a little looking, and there are a couple of outfits out there that do reprints. NB: Any FSM 1987-1990 should work for your 1990, but note that ChryCo did change the colouring on some wiring mid-1988. Apart from that, everything applies - just note the change if you happen to score an "early" Chrysler FSM or actually manage to nail down the (huge!) AMC FSM.
Any questions? As you can probably tell, I've been at this for a while (and I've probably caught most of the questions that get asked about RENIX...)