^hey I live in Westchester (right next to you) and I am realitively new to the area too.
I have been looking for someone to hit the MTB with...we'll have to set up a ride soon!
Now to answer your question Westchester Bike Shop (on 22nd St across from the Giordano's) is primarly a MTB shop and I have stopped by once and talked to him. According to the owner there are all kinds of trails throughout the Salt Creek woods (off that nice blacktop trail).
Most morning's I ride the blacktop on my Road bike and I see all kinds of single track off of it...
The best in the area is suppose to be Tinley Park...I ride with a group out of Wheaton and they head down there often. I haven't ever made it. I'll let you know when they are going next.
Welcome to the neighborhood!