I think my dog has seperation anxiety, HELP!!?

My dog is a 8 mo.old boxer/lab mix and everytime we walk out the door, even for a second, she goes crazy. She gets on the tables and gets stuff down and chews. She goes after anything that she can fit in her mouth. She has even chewed up the cushions to my new couch. Since she was a puppy we have always had chew toys and rawhide bones. I would like to leave her out during the day but I dont trust her, so she has to stay in a crate. Any advice would help