Yeah, Losi makes a brace to go across the rear hingepins.. are you running that?
# LOSI A9951 Rear Brace, 3 deg, xx-4 & X - 5, blue alum, $4.
http://www.xfactoryrc.com/storelosi.htm
Atomic Carbon also makes a rear brace, though you have to notch the back of the chassis a slight bit to make it fit - see Jimmy's X - 5 review.
http://atomic-carbon.gforceimages.net/products.htm
The rear braces can be a pain to keep on, so there has been some discussion (a while ago) about different solutions. What I've done is replace the rear hinge pins with shock shafts, oriented so the threaded end is out the back, and put a lock-nut on them which captures the brace (I don't run a rear bumper). Another solution, well-written up, is here:
http://www.one-ten-rcforums.com/4wdr...ic.php?id=1329
Hope that helps some.
IIRC Brian is running a 110 spur, which is pretty big when you look at it.