Struggled with a bad back this year, getting more miles in on the road bike, plus new bike, plus a mis-guided attempt to change my position for a slightly less aggresive one.
Talking to an old mate who I used to race with, he was convinced, and I agree, that more upright positions can cause as much problem as a too low one, and that a shorter more cramped position certainly becomes very uncomfortable for me.
I used to try a short stem, but my own tip would be to stretch out more.
I've always used an XC style position on the mountain bike, but the current trend for stubby stems and wider bars improves control but leaves me sore.