To the OP:
I have 37" KM2s... But I did all the proper trimming + pinch-weld hammering and I run Glassworks fenders. There are a few guys on here who are running 37s with stock fenders (E63 and I think bstoner59) BUT they are using stock wheels which do not have the offset you're running. They both also offroad hard and seem to be doing fine with that setup, but again, stock wheels, so it's a different ball game than what you're talking.
With the offset you're running I strongly urge you to heed the advice TMX posted in the long post above, he summed things up very succinctly. Either buy aftermarket fenders now, or trash your stock fenders and buy aftermarket fenders later.