John:
Go to this web site:
[www.russellbrown.com]
It has a large number of free video tutorials by Russell Brown, one of Adobe's top Photoshop people. There is one about half way down titled "Hidden in the Shadows." The best tool to use for doing what you want to do it the Photoshop curves tool. This tool lets you manipulated the highlights and shadows independently. Conventional darkroom photographers have been doing this using different developers, dodging and burning while printing, etc. Make a copy of your scanned image and work on that copy, so if you screw up (and you will until you get the hang of it, your original scan is preserved.
The following web sites also have free Photoshop training:
[www.photoworkshop.com]
[www.adobeevangelists.com]
[www.adobe.com]