You can do Miller. I've been up it with stock Disco's -- even with stock tires! Might break something, dent something, but that's all part of the fun...
Don't do it alone though, imho. Lockwood (the access trail) is also terrific. Beware the rock climb just at the start of Lockwood, just past Peru Creek. I'm one of many who has battle scars from a nasty rock outcropping on the creek side of the climb.
There's a trail run up that way soon -- Aug 27th I think is the plan. It's SoCalRovers, a Yahoo group. You can join up on Yahoo Groups.
Also check out www.sclr.org