Particularly in the rear. when I have my jack up all the way on the rear diff (>18"), it is still not enough to get the stand under the stock jack point (the jack stands are minimum 11") I don't see where else to put it. one jack stand under the diff seems too unstable. could i put two directly on the axle?

 

First, since you're lifting the rear wheels I hope you're using some good wheel chocks on the fronts. Then, lift the rear as high as you can, put blocks or ramps under the rear wheels, lower onto the ramps, and lift again with a block between the jack and rear diff. I use a large shop jack, so the saddle is pretty big. If your's is small, I'm not sure if I would recommend the extra block between the jack saddle and diff.

Never get under the car with just the jack holding it up, and always try to shake the car once it is on stands just to make sure they won't slip while you're under there. Be safe and good luck.