Is it a slamming sensation or are you just getting tire marks on the fender? The heavy duty bumper helps, but I was always had a lot less tire marks when I ran the shorter pull rods from Pro Circuit. I'm currently not running them and have noticed more tire marks again.
Also, is the bike balanced? I would think the forks would be a problem too. If they're not, maybe you could lighten up the fork a little bit either with a lower oil level or less clicks on compression to take some load off the rear when bottoming.
Just my .02!