First, welcome to Pet Talk! I'm sorry to hear about Darby's fear aggression. It's really hard to tell if this can be trained out of him, but I think it's worth a shot. Since he is a rescue, this behavior probably stems from previous abuse. Do you know anything about his background?
Consult with a canine behaviorist/trainer. They should be able to give you some advice.

**hugs**

P.S. He really is a cutie, no wonder people want to pick him up! He sort of resembles my little Priya!