The vacuum disconnect was probably a "fuel economy" thing, and done for federal CAFE standards. It was done away with a little later, retaining the NP231 (and the NP242 never had it.) As long as everything's in good shape, ditching the disco should be no trouble - I just "forced" it over of my 87 and forgot about it - and quite a few others have as well. You should have no trouble if you go to a "non-disco" axle up front.
If you can get an automatic donor vehicle, you should be able to score a pair of axles geared 3.54/3.55 - and that works fairly well with 30's and 31's. I'd say 4.10's would be a better choice for a road/rock rig, but they're a little harder to locate, and I think both carriers "break" at about 3.73 - meaning you'd have to get a carrier of some sort as well as gears (and doubling or tripling the cost of the regear.) At that point, it would probably be worthwhile, since you're doing the entire differential, to think about adding your LSD/Lockers while you've got it torn entirely to bits. Detriot, Eaton, and ARB all make lockers that replace the carriers - and there are probably more that I'm not thinking of.
If you ask intelligent questions, you (probably :laugh3: ) won't get blasted. However, we get our share of questioners that seem to hail from Moron Mountain, and it gets a little wearing.
One of the best things you can do for yourself - here or anywhere else - is to remember that we will probably never see you - except by what you write. Which means you are going to be judged not only by what you say, but in how you say it. What does that mean? Take the time to think about what you say before you type it. Make it look nice - that makes it easier to read. Polish your grammar and spelling, if you think you need help. Follow to standard rules of writing formats - they've been around as long as they have for a good reason.
Remember that readability is key to interaction in any written forum - like this - and you can enhance your known reputation simply by treating spelling, grammar, punctuation, and formatting as serious. I'm not trying to say that you're doing anything wrong, but it seems to me like you might be fairly new at this sort of thing (written format interation - not an opinion of your mechanical skills) and I'd like to give you a few pointers.
It's also worth remembering that an occasional "typo" won't be a problem - we all do that every now and then. But, the more polished your writing becomes, the more likely you are to get good, useful answers to your questions, and the better you'll feel about the way people write back.
I've been thinking about this for a while, and I'm trying to work out some sort of "netiquette" thing as it applies to the written word. None of us are at all perfect - but it shows if you think about what you say before you say it, and it gets you much better responses. You'll see that anywhere you go!
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