Having done the Onion to Whitney hike myself, I think you'll find yourself deciding to skip Williamson. It's both out-of-the way and adds an unnecessary fatigue element to the equation.
If you don't want to traverse x-country from Williamson, then a much more straightforward & logical hike would to still go in @ Shepherd. Williamson (or Tyndall) are right there. Thereafter, the Shepherd trails joins the JMT @ Tyndall creek, making it a more logical continuation on the way to Whitney.
PS I've driven my Sienna mini-van 3 times to the Shepherd hikers' TH without any problems. Just watch out for rocks.
Am in complete agreement with "Hobbes" on this. If you want to do Williamson, then do Tyndall also seeing how you are there and it is the more enjoyable climb of the two. If your goal is to do Williamson no matter what, then I would also recommend coming into the area over Shepherd Pass. But if you do start at Onion Valley and want to get over to the Williamson/Tyndall options,once you are a mile or two south of Forester, LEAVE the JMT right as you get to the south face of Diamond Mesa at the 11,800 elevation. Cut under DMesa and head XC to the tarn North of Tyndall. In doing this you save 3 miles or so, which you will appreciate latter on in your trip. Once you head back to the JMT, take Wallace Creek up to Tulainyo Lake and get over to Upper Boy Scout Lake via Russell -Carillon pass. This gives you the choice of doing Rusell instead of Whitney, which I would suggest 95 out of 100 times. I have been to the Williamson Bowl area multiple times via both Shepherd Pass and starting at Onion Valley, and have to say Shepherd gets my vote. Williamson,Tyndall, and Russell would be three 14,000 foot peaks, and an epic trip. I have climbed all three, trips starting both at OV and up Shepherd, and no matter which starting point and peaks you end up doing, you will have the time of your life. Good Luck.
P.S. Another thing Hobbes is right about is that getting to the TH for Shepherd should be doable in just about any car as long as you drive smart. 4WD and high clearance is not mandatory.
