I have personally run rear shafts as short as 10" before. With proper angles, what is the shortest, workable, rear driveshaft you have seen run? This is for mainly off road applications.
I have seen about a 10" shaft work ( that didn't sound right ). Personally, I would want one longer. Thats one of the beautiful things about a Commando, my rear shaft is +/- 36".
I'm guessing you are thinking doubler for your Jeep. I think the shorter you go the less droop/flex you will be able to engineer in it. My thoughts are to call a place like Tom Woods and find out how much angle can be put on one the length you are gonna have to work with.
2nd, I think it would be a grand time to lengthen your heep a bit.