A friend of mine does the rock crawl jeep trail thing and he has after market bumper that are pretty rugged like yours and he had the same problem. Drilled 3/4 holes on the underside of each corner of the front and rear bumpers and we fabricated a bracket that slips over the contact point of the jack with a 1" h x 5/8" d pin welded to it and is held in place with a detent ring pin with a lanyard like this kinda (
http://www.pivotpins.com/catalog/Detring.shtml ) through the hole in the jack. The pin is tall enough that it will handle quite a bit of angle and definitely wont slip. and if you need to use the jack without it just pull the pin slide off the bracket and you have stock jack surface.
here's a crude 30 sec drawing that I hope illustrates what I am talking about
Attachment 1225