You'll have to build a temporary brace for which ever post is giving you problems. W/O seeing it, you'll probably want to run two or three 2 x 6's, on their sides and secured to each other and centered under the area you want to brace, over a distance of say 10'. If the 'area' that you'll be working in is close to the house, then one end of the 2x6s will be supported from UNDER the porch and the other outside-of/away-from the porch. Toe-nail the 2x6s into 4x4 posts which sit on a base that's something like a 2 foot piece of 8x10. Then, angle-brace the 2x6s at a 30-40 degree angle to 2x4 stakes. In other words, when you're looking along the 2x6s, you have a triangular-looking support structure. (See the e-mail I'm sending you.)
Another options is to support the porch directly with 4x4 posts. You'll need to set a 'base' on top of the post(s) so they span at least 2 joists. Use at least 2 2x6s between the TOP of the 4x4 and the underside of the joists. Make sure the 4x4 is sitting on a 2' 2x10, or similiar, on the ground.
When you're all done, fill your hold a few inches at a time, and tamp with a tamper or a 4x4. Don't put too much dirt in at a time because it won't pack right. Tamp real good. When you're finished, you basically re-dig a hole for a NEW porch post. I'll leave the details up to you. THere are a LOT of books and sites on the WEB that show how to do a deck/porch post. Of course, you'll need to work on leveling the porch first. DON'T cut the new post too short until you're ready to attach it to the porch itself.