All really good responses to your dilemma. I will add that HEET is the usual stuff to use in the gasoline tank in order to try to absorb water in the tank, but ACETONE is 'mo-betta. Use a 'turkey-baster', insert long tube end into the tin gallon container that the acetone comes in, squeeze the bulb end, and you should have about a couple of ounces there-in in which to squirt into your tank filler tube. If you have ice in the tank, well then, not much will help except for manually removing said.., yeeech, or allowing fuel/ice in tank to warm up totally somehow. Not really your problem, perhaps, right now, but it is a preventative measure to anticipate water/ice in tank, or in your fuel line somewheres plugging things up.
At minus forty-seven degrees f., I've placed a pot full of nails, and old motor oil into it, i.e., full up, and lit it off. Shoved it under the oil pan for a half hour before the crankcase oil was warm enough to allow the starter to turn over easily enough to gain a start-up. The 258 engine I did that to was free of any oil, just oem paint. However, my 4.0 engine is oil-mud coated, and I dare not allow a flame to burn down my XJ. Now that I have electricity. I use:
http://www.moroso.com/catalog/categorydisplay.asp?CatCode=15014 At the bottom of the page click on the instruction button(s). (get 2, if possible.., as they do die). I'd rather use an engine block model that goes to one of the freeze plugs, but that's too much a hassle straight away. However should you ever have to do some major work in that area, that's a good time to introduce said model, as they are bullet proof. There are some models that splice into your coolent line(s), but I refrain from them. one of the moroso heating pad models has a short lead from it, so you will need to have, say, a shorter extension cord that you can tie off somewheres up front of you XJ, in which to utilize another extension cord from your main(s). outlet. The model with the longer 36 inch lead is just about right. Call around to your auto parts stores and see if any has them. Push come to shove, you can order from say; summit racing, and have it sent to you next day delivery via FedEx, or UPS, for examples.
Once you have that done, you might consider a battery tray ceramic heating pad, and maybe even a battery electric blanket that surrounds the battery in conjunction with all the other great ideas given you to overcome your issue.
-30 f. is nothing to mess with, so you might try alternate ways to get around. No point in getting second, or third degree frost bite, in which means losing a handful of digits in the third sense.., which makes no sense at all when it comes to dealing with a vehicle that is down. That is why God invented taxi's, buses, and friends. However if you torch your '89 XJ, it will keep you warm for a while.
8-MUD: Really like the idea of drilling a hole into the intake rubber tube for the introduction of starting fluid, and so when it warms up enough to play around, that is something that is a handy technique for us all should such a need arise. I have not seen too many rubber plugs lately, but I am sure they exist, lol.., are the rubber plug owners moving the factories over to china, and we have to wait for production to produce them, and to ship them Stateside? Whatever, lol, we in Alaska do not have the variety of pieces and parts the rest in the lower 48 share, so we have to do a lot of inter-net shopping in which to obtain just about anything. Here, NAPA took a month and a half to shuffle a part through their 2,000 mile, (plus), logistics system to get it to me. Would've been faster else where via the net, and U.S.P.S.