2 or 4 Wheels Alignment

Is the rear Caster and camber adjustable for 1999 Avalon XL? Should I do front wheels or 4 wheels alignment?

I doubt it. At best there may be an adaptor kit that lets you adjust camber if it's way out. Check your maintenance manual in the Factory Service Manual thread at the top of the forum.

Generally always have a 4 wheel alignment done unless you've just changed suspension components on the front and the rear has been aligned recently.

4 wheel.