Woody,

I have been searching the web for plans to build a shed for storing
lumber that is attached to the outside back wall of my two car garage

Look for a storage shed in one of the magazines. I seem to recall
a storage unit, like you suggest either in Wood or Workbench within
the last year. Also, go to a book store or Woodcraft (which has
a lot of great books), there's a book on storage sheds (Sunset?)
and I think one of them has what you're looking for.

Good luck!

MJ Wallace