The San Fernando Valley has two big yards that I visit every couple of months. They each usually have a lot of Camaros and Firebirds, and "scores" are usually always excellent. It's been a couple of months since I've checked them out, so now is probably a good time to pay them a visit. Over the last couple of years, I've probably pulled enough very nice parts from these two yards to build an entire car.
One's a PYP(Pick Your Parts) on Pendleton at Glenoaks, and the other is a UPP(U Pick Parts) on Penrose at Telfair. They're only a few blocks from each other in Sun Valley, right off the 5 freeway.
More "locally," there's a private yard just before you get to Mojave. It's just kind of... out there right along the 14, on the east side of the freeway. You'll have to take dirt roads to get to it. I've only been there once, but, surprisingly, there were more thirdgens than I expected to find.
The Ecology in Lancaster(on 90th St. East) rarely has anything decent or fresh. But I've occassionally found something good there, so it's always worth the $2 walk-thru every few months. Just don't take along any expectations.
Unlike Lancaster, the Ecology in Adelanto(on the 395) usually does have a decent supply of Camaros and Firebirds. But like Lancaster, the stock doesn't change very often. But I've found good things there, too, so it's also worth a visit every few months.
On my last visit to Adelanto(maybe a month ago), there was a 4thgen with a PBR disc rear. It wasn't a posi, but the PBRs were excellent. Someone had pulled the rotors, but left the backing plates and the PBRs!

They're probably gone by now, though.
If you go to Adelanto, but then you feel like going for a bit more of a drive over to, and down, the 15, there are two good Ecologies close to each other: one's in Rialto, and the other is in Fontana. I suspect one of those is the one Drew is talking about in his post.
Happy hunting!!!