Where and when does he get "jumpy" and nervous? is it when he's out for a walk or meets other dogs or people? Finn is a bit skittish and i've noticed when we go out for a walk at night, he is more apprhensive. I attribute this to the car lights that seem to be coming right at him. Has Tank been checked out by the vet lately? you may want to do that since you mentioned him being a little more withdrawn recently. If everything's okay w/him physically, i don't know if crating him would be the best thing since you don't want to encourage him to be more withdrawn or isolated. Of course, the crate can always be there for him to retreat into but not so much as a substitute for socializing.