I'm not a Vet, but it sounds like he has some seperation anxiety. He's doing the bad behavior to get back at you for not being around, and then you yell, so he does another bad behavior (this one even worse). My dog does this, but without the peeing after we yell. I know what you mean about the slinking and crouching, my dog does this when he knows he's been caught in the act - not because he has been or is going to be hit. He just knows he wasn't supposed to do it and did it anyway
Anyway, most vets have a behavioralist (sp?) that they work with. It would probably be worth the money. We got a private trainer that also dealt with behavior (6 weeks, $45/session) it was worth it. Also, you could look into doggy daycare once or twice a week, in ATL it's about $20/day (we do this too).