I did my rear ones a few weeks back and had the same trouble. In the end I used a Propane torch (didn't have anything hotter) and heated the knuckle around where the bolt goes through. Since it was -10*C outside at the time, I heated until I couldn't hold the torch steady anymore, or about 5 to 6 minutes.
Next I sprayed some good penetrating oil in every crevice I could, then I put the nut back on leaving some space from being tightened down all the way. Now take an impact socket and hold on the nut with one hand and then hit it repeatedly with a ball pean hammer - it could take some convincing. If it doesn't work the first time...repeat the proceedure. Once you get some movement of the bolt, you might try turning the bolt back and forth with a good power bar, again using penetrating oil liberally.
Don't give up, you'll get it