I have tried to draw my situation. I need to vent a laundry sink (which is is the basement) an I do not have the option of running a new vent to the attic. So must I use an AAV, or can I vent into the main stack even though I am downstream from a toilet? Doesn't seem like I can go in to the main post-toilet, but I wanted a knowledgeable person's answer. I appreciate any help you can offer.

Louis.

http://dl.dropbox.com/u/685086/plumbing.jpg

can I vent into the main stack even though I am downstream from a toilet
The very fact that you're asking that question tells me you already know the answer. Where would you connect the drain to? The same stack? But any how you can't do that.
Code dictates that you can not discharge a major fixture past a unvented minor one and that's what you'll be doing.
Drain into that stack but first install a AAV just downstream from the trap,(see image). Good luck, tom