I don't think there are any salmon or steelhead in the Yosemite region. The regular trout fishing isn't even particularly good there, by California standards (much less Pa. standards, which are higher overall in my opinion).
The American River has a decent run of steelhead and salmon. That's about the closest winter fishing for salmonids to Yosemite, which is in Merced County, about 3 hours away of the American River. The Lower American River flows between Sacramento and Folsom.
Best place to find info on Northern California fly fishing is http://www.kiene.com/forums/
I fished the East Fork of the Carson River in October. South and a little bit east of South Lake Tahoe, about 30 miles away. The wind was torturous. 50mph gusts. Maybe the worst fishing outing I've ever had. And I wasn't even skunked- I caught one 12" rainbow, a sterile put and take plant. Miserable looking. It was all but impossible to fish, even by letting the line out into the wind. The wind was so bad that it pushed the leader along like a sailboat sail, towing the fly beneath it. When I could get the fly to land on the water, that is.
I had scouted the river the week before. Since I couldn't find the sign indicating the route to the river (it's pretty obscure, and not at all where you'd think it should be), I just parked the car and headed up the hill due East, knowing that it had to be on the other side. That turned into a little project. Finally got there
Then I had to get back over the ridge before sundown. Another project
Eventually made it to the crest of the ridgeline.
Around 700 vertical feet up and 900 feet down, according to the topo map I found on-line.
The next time, when I brought my fishing gear, I found the trail. That was easier, but still no carefree stroll. Took about an hour and a half to go 1 1/2 miles. Felt like a lot longer.
Pretty enough place. But I was expecting a wild trout excursion, and I had thought the stockies were confined to the other fork of the river. I also underestimated the wind. I'll never return to that place if there's more than the hint of a breeze.