I gotta runto work, so will reply in more detail soon, but the first thing you need to do is order the book "Mine! A Guide to Rescource Guarding in Dogs" by Jean Donaldson PhD.
It describes your dog to a T (and my foster-turned-permanant rescue Berner, Charm) and has an excellent desensitization (gradual counter conditioning) program that you will want to start working on ASAP. It is a positively based training program that does not use aversive methods (which could potentially worsen the aggression.) This is an inexpensive little easy to read paperback book, available from www.dogwise.com. Vilma Briggs (Kistner) Mt. Gilead, OH U-UD Mocha Java Slurp, UDX, HIC, TT Ch. Brighteye Expresso Bean, UD, NDD, TT U-CDX Our Little Buddy, UD, NA, TT Thirdtym's A Charm, CGC and Ted