You're almost certainly talking about El Dorado County. As you continue up the highway to the north and east through the heart of the Shenandoah Valley and most of the Amador wineries, you cross the county line and find first the Fairplay appellation, a fairly recent addition to the AVA list. The counties in the Sierra Nevada foothills run mostly east west, so in order starting from the south, you have Calaveras, Amador and El Dorado. There are wineries in the counties further north and south but most activity is focused in those three.
In El Dorado County, the wineries are actually mostly clustered around U.S. Highway 50, the next trans-Sierra route to the south of I-80. There has been an explosion of winery activity in the area, which seems to show particular promise in growing the Rhone varieties. And yes, there are some very good wineries. Probably the best known and one of the "oldest" (the oldest establishments date only from the 70's, most are much newer than that) is Sierra Vista, which specializes in Rhone varietals (but does other things, too). In addition to the Rhones, they make a nice, crisp, unoaked reasonably priced Chardonnay. Two other labels you may have heard of are Lava Cap and Boeger. The latter seems to specialize mostly in Italian varietals in addition to the inevitable Zinfandel; their Barbera is usually good. Steve Edmunds of Edmunds St. John, whose winery is in the Bay Area, has made a name for himself in Rhone varietals and buys much of his fruit in El Dorado County.
That's not to say that everything from El Dorado County is wonderful, of course. As in any region, there are reliable producers and not-so-reliable ones, excellent wines and awful ones. I left the area a couple of years ago and with all the changes, I'm not the best source for which are which. Best just to go taste for yourself, I guess :) You can read more at the local winery association's PR site at
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I hope that helps!