To get the most bang for the buck, I'll probably do something like this: (please comment)
1) Front Diff Cover ~$100. Teraflex has one with a drain plug, which would seem to be a convenient feature.
2) Rocker Slides ~$325. Wow, these are expensive. I really don't want to drill into my tub, but it seems necessary. Any suggested models for a YJ?
3) Rear Diff Cover ~$125-150.
4) Oil Pan ~$325 (another expensive one! Not many choices, which makes me wonder if it should be moved lower in priority...)
Just putting these down lower, since the Jeep already has some protection in these areas.
5) Gas Tank
6) TC Skid Plate Replacement - would probably try to gain more clearance if I spend the money on this one.
The steering gear box protection and U-bolt protection could be done at any time - those costs are pretty small.
You're getting a lot of good information already, so I won't repeat it for the sake of posting something. A couple of points that come to mind though after reading everyone's comments. While most are good, I don't think there is a better diff cover out there than what Dan makes at Ruff Stuff. Priced right, simple, and bullet proof, literally. AND he's a JF vendor/supporter. I'd avoid those with drain plugs as someone mentioned. Just another potential failure point when dragging across rocks. Its my opinion that if you need to drain your diff fluids, then you should probably be taking the opportunity to inspect the internals anyway.
Last point, and not knocking the Rubi, but you already have a great platform to begin with when you want to build up a crawler. Save the newer purchase for a nice street ride. Build this YJ and beat the hell out of it. They love it! Promise!
As a followup, I went to a local offroad park (Hidden Falls) with a bunch of other Jeeps yesterday. I'm the low-rider white Jeep.
I took off the nerf bars before I went. I learned a lot yesterday. PO had a receiver hitch on there, and I need to take it off. I hit it hard on a ledge. Also hit the gas tank, as someone predicted. Didn't come close to hitting the rockers yesterday (thankfully), but I could see how that could happen. Was actually able to go everywhere that all the other Jeeps went - it's actually amazing how capable our stock Jeeps are.
Good buy! I'm thinning the d30 may have a new friend.
I'd suggest a gas tank skid. A few weeks ago I came off a ledge and smacked my gas tank. I wasn't thinking about it because of the skid but I filled up the next morning only to come out of the house 20 min later to see a steady gas drip from an 1.5" crack on the back side of my tank. $300 later I'm back in business with an MTS tank that I'm not happy with.
Adopt your pets. Don't shop for them.