seriously, check your speedo gear! i did some work on mine and wasnt paying attention when i was finishing up. i didn't line up the housing(with the gear #'s on the side) properly. the result was at first a complete lack of response from the tachometer. i didn't mind much because it didnt seem to effect anything serious, and i wasn't putting miles on the odometer (i fully intended to fix it the next time i worked on my jeep however). eventually the tach started behaving irratically... 0 rps then suddenly 5000 rpms with no apparent direct cause or at any predictable moment. also, my idle became more erratic with the increasing frequency of the sporadic tachometer. anyway, i finally took ten minutes to replace the speedo gear with the correct size (i had regeared recently) and properly aligned the housing. both the idle and the tachometer were instantly remedied. weird. i'm not sure how the speedo gear had any effect on my idle, but whatever. it worked. if you havent messed with your transfer case or dropped it on any rocks or stumps recently, this probably isnt your solution, but i thought i'd share.
