Jeep Wrangler cranks but won't start?

I have a 99 Jeep Wrangler, manual transmission, 130k. I left it in a parking lot for about a month, and on the night I drove it home, it died at a stop sign. It didn't shutter out and die or do anything all that crazy, it just sort of shut off. We were a little low on gas, so we got some gas, and some dry gas and it didn't do anything. We also weren't able to pop start it. We had it towed home and haven't been able to come up with anything yet. We had one of our "mechanic" friends (he's not always the brightest), and he concluded that there was no spark. The Jeep cranks, but will not turn fully over. It does everything but start, it will come as close as possible, but does not turn over.Recently we tried replacing the cap and rotor, no go. The plugs and wires need to be replaced, but not in any dyer way, it's not the problem. The suggestions I've had are to replace the crankshaft position sensor, ignition coil and ignition module. I'm looking for where to start, as I don't have money to just dump into guessing parts. I think that's all the pertinent information. Thanks so much.