I have a vacuum leak issue since putting a new engine in that i've been looking for and as a result, my four wheel drive won't kick in. The light comes on but it won't engage. Anyway, this morning i got stuck in about a foot of snow in a circle, while i was trying to get out, the 4 wheel kicked in momentarily and i got out. After I got out, i went and ate when i came back out and tried to start my 91 cherokee 4.0l auto, it wouldn't run, it turned over fine just wouldn't stay running, like it was trying. I waited a bit, then tried it and it started limped it home and it died twice while I was getting there. When i got home i thought it may be some condensation or something so i tried to let it idle for a while but it wouldn't stay running for very long. It starts and idles steady then every 30 seconds or so the tachometer drops all the way down like the engines quit then the engine kicks back in and the idle revs it back up and then it repeats, not steadily but it repeats. I have a freind that said it may be the alternator, but i have no clue about the vacuum issue or this. Help anyone, please!