An 87-90 4.2L YJ should have 3.07s stock. I have seen a couple with 3.55s but that is not common. You have probably noticed its a dog with the 33s. If I were you I would recommend finding a set of stock YJ axles from a 4cyl as they will have factory 4.10 gears in them which will work great with the 33s. This will keep everything a bolt in so you can enjoy your Jeep and learn how it does off road. Expect to spend $200-300 on a set of working used axles, $50 for gear oil and a tube of RTV (to change the fluid in both axles), and $80 on u-bolts to do the swap. If nothing else breaks and everything is working you can bolt all your existing stuff (shocks, vacuum disco, brakes, etc) to the new axles and be done in a weekend. Soak every bolt in PB Blaster every day for a week before you work on it. Leave your front sway bar hooked up but unbolt and remove both the front and rear track bars. You will see a good improvement in ride comfort and flex and it has no real effect on how it drives.
If you have junkyards in your area check them for YJs that can be the donor. Also check craigslist.org and car-part.com for even better deals.
Maybe the mods can move this post to the correct section so you can get some more feedback.