I have a relatively new Sears Craftsman riding lawnmower. I accidentally ran over a protruding rock last year and ever since then no matter what level I set the blade at, it scalps the ground down to the dirt!

I hate to take the mower to a lawnmower repair shop because I know it won't be cheap.

Should I replace the blade and does anyone know of a diy manual that can guide me on how to adjust the blade?

Blades on mowers like yours that come with the mower as original equipment or that you purchase from Sears are designed to bend when you hit certain kinds of objects such as rocks, roots, even limbs or pinecones sometimes. This is prevent damage to the engine unlike larger "tractors" whose engines can normally withstand such impacts. It sounds as if one or more of the blades might be bent? Have you looked ? If bent, then simply replace them.

I bought a Sears Craftsman LT 3000 last year and had to replace the blades twice in the first week. After that I bought one of the roller kits that attatches to the front of the mower deck and since then, no more scalped lawn and no more bent blades. I believe it should have come with my mower as my old mower never bent a blade on my lawn. The roller kit was the only solution if I wanted to keep my mower and my sanity.

No adjustment of the blade, just the fact that you have severely bent the blade, and left unrepaired is going to end up with someone hurt, or doing damage to the spindle assembly that holds the blade. Replace the blade.