Isn't most of it TNF? That's open, if you can get to it.
I assume you are coming down 80 from Tahoe? You can get off at Colfax and take Iowa Hill Rd. down to the N. Fork of the American. I believe there's a campground there, but don't know status. Yankee Jim is another interesting crossing of the N. Fork, which you can catch just west of Colfax. Cool suspension bridge.
I've heard there's excellent MTB at Sugar Pine Reservoir, which is also used by the moto crowd. Haven't been there, but my buddies used to ride up there on the trails around the lake.