Dave

There are 2 repeaters that might work depending upon exactly where your going to be in Yosemite;
Mt Bullion on 462.600 (located on Mt Bullion) has limited coverage into the valley
NVGMRS-.700 on 462.700 Located on Mt Oso has better coverage into the valley but still limited
 
You can look both of these up in the Repeater listings, both require permission.
NVGMRS-.700 is your best bet and it covers into Sacramento as well. Being located on Mt Oso it has great coverage of the entire valley and into the foothills as well.