This is not a 100% comprehensive step by step install, but combined with other information out there on doing a 4 bolt swap, this should do the trick.
Let's start with the obvious... We're KEEPING 4WS, thus we are keeping the same differential case. This means we are keeping the same pinion shaft, and thus, the same ring gear (as they're matched units, even the FSM states never to mix and match them... kind of like main and cam caps).
First thing, get the back side of the car in the air. You're going to be spending some time under there.
After you get the car secured with jackstands ON THE UNIBODY, NOT THE SUB-FRAME, drain the rear diff, and remove the axles. In order to do this, just remove the 3 bolts on the outer end of each axle, then pry each axle out about 1" with a prybar, then they should each pull right out.
Now that the axles are out, you're going to need to drop the exhaust, at least in the rear of the car. I just pulled the 3 rubber hangers near the muffler, and it gave me the space i needed.
After the exhaust is hanging there in the breeze, take the 2 bolts out of the mustache brace, and drop the entire diff/frame assembly down far enough to get at all the bolts that hold the rear diff cover on. Once you remove the rear diff cover, things should look like this:
If you have a stuck parking brake cable, now is a good time to replace it. I don't think you can really do it without dropping the frame down at least as far as it is shown above. It was still pretty tight when doing it with things as shown above. Anyhow, moving right along...
Now that you've got the cover off, mark the bearing caps for each side, mark which side is top/bottom, (do something so they cannot get mixed up, i just remembered how they went and placed one on each side of the diff cover) remove the 4 bolts holding the diff inside the case, and remove the differential assembly. Do as you wish with the bearing races, but KEEP THE SHIMS and make SURE the one on the right, STAYS on the right. Same goes for the left, obviously. You need -these- shims for reassembly.
This is the 3 bolt rear diff.
You need the ring gear, the gold 4WS pump drive gear, and the roll pin which goes in said drive gear off this assembly.
Remove the ring gear. This part is simple. It's simply bolted on with a few bolts (14mm socket and 1/2" impact gun recommended)
Now comes the bitch of it all... removing the bearing in front of the 4WS pump drive ring gear and the gear itself.
It doesn't really matter how you do it, but you've got to get the outer cage for the tapered roller bearing off the diff, to allow the gear to slide off. I used a cutoff wheel and carefully cut through the cage.
Two cuts on one side, and one cut on the other, so it's easier to bend away and let all the rollers loose.
Now comes the fun part. You need a press and bearing splitter, or go the route i did. I went to Shipwreck Freight and got a 2 jaw gear puller.
This gear has not been on all that tight on any of the 3 of these i've done.
I didn't manage to get pics of the removal procedure, but it's not that difficult. BE VERY CAREFUL not to mess up the gear. It's actually easier with the right press and bearing splitter setup, but not a lot of people have a press available all the time.
Once you get the gear off, there will be a little roll pin left in the diff. Remove this pin as well, and if you bugger it up a little bit, don't worry.. you can open it back up with a chisel or screwdriver and get it back to the right size.
Things should look like this when you're done.
You now need the differential out of the 4 bolt case. DON'T MIX UP THE BEARING RACES! Keep them with their respective bearings, or write on them with a sharpie like i did. Once that's out, you need to remove the ring gear. You also need to CAREFULLY remove the bearing (complete) on the small side of the diff. You'll be re-using this bearing, so don't mess it up!
Once you've got the bearing off, you can put the roll pin in the differential, and put the ring gear on. I used an axle cup to get the ring gear on evenly, since i don't have a press yet. I just have the bearing sitting on the edge of the diff in the 2nd pic.
Put the ring gear out of your original differential on the 4 bolt diff. Make sure the bolts are clean and dry, and use loctite when you put them back in. (you can smell the loctite when you remove the bolts initially... this should be a clue)
Now you just have to get the bearing back on. I ended up pulling the inside race off the 3 bolt rear, and using it to help press the 4 bolt bearing back on. Make sure you don't go beating on it to get the bearing on with the opposite side sitting on the ground or something. I used the 3 bolt diff to support the 4 bolt diff and keep the other bearing from harm's way.
Once the bearing is pressed back on flush, you can put the differential in the case. It's not "easy", really. I actually put the diff and one shim in first, then tapped the other shim in. Once that's done, you need to get the original bearing caps back in, on their respective sides.
Now, it's a good idea to check and make sure your differential back-lash is still in spec. I measured mine before disassembly, and again after reassembly. There is still the same exact amount of back lash. This has been the case with all 3 of the 4 bolt swaps i have done like this. If something doesn't match up, you may be in for some serious work. Setting back lash and tooth contact is no easy feat. Fortunately, the CASE is the variable in these differentials and the diff itself is a precisely machined unit. Case variances are where shims, crush washers, preloads, tooth contact, and backlash are affected. You -should- be able to follow this procedure and have everything line back up properly.
Now put the diff cover back on, sealing it with Ultra grey, of course.
Bolt the assembly back up into the mustache brace.
Swap the axle cups (27mm socket) and install the new axles.
MAKE SURE the circlip on the LSD (right) side axle is centered when you put the axle in. Use some grease if you need to, or you can give the circlip a SLIGHT bend on one side, and center it in its groove. This last one i did, i actually didnt do this right, and damn near messed up the circlip pretty bad. It took some doing to get it bent back straight, greased up, and put in the diff properly.
Once it's all back together, fill the diff with fluid, double check and make sure everything is tight, and enjoy your new LSD 4 Bolt on the road!