I'm curious to know how too.
In the mean time..you can try and add the shadows to the ground texture. I think alot of games do it this way. I say this because I've played games where the trees move but the shadows don't. I'm sure it won't be an easy task to make it look right (or acceptable) but at least it's an option.
Don't be satisfied with what you can do but rather strive to do the things you can't do!