MY 6 year old Dalmatian is behaving weirdly?

  He refuses to get out from underneath the desk, and whenever anyone gives him attention he shakes and wags his tail really fast. He's never acted like this before, should I take him to the vet?
Low fast tail wagging is NOT happy but can be anxiety, submission where high fast tail wagging can be a sign of aggression or dominance so don't go by that. Do you live where you may be expecting a storm? Dogs are very sensitive to climatic an barometric changes and can predict tornado, earthquakes and storms. Just one option of quite possibly thousands. Any thing else change in your environment?
yes, when your dog has a change in behavior, you take them to the vet. We could guess everything from lyme disease to seizures and that isn't fair to you or the dog. Take him to the vet. It could also be some kind of stress like from vision or hearing loss, it just isn't fair to guess things. Good luck.
I don't think it anything to worry about. Sounds like he's having fun. A dog's way of saying ha-ha. If it's bothering you, watch the dogs behavior right before and right after he hides. He might be trying to tell you something. Hope it works out ...
i got a dog that was abused and she was like that for a long time don't know i that's possible in your case that someone hurting your dog but it Sounds like how my dog was
About the tail wagging part, he's just showing happiness, not a problem. pets question and answers,www.5d2d.com

Although for the desk - I would coax him with his favorite toys or treats. Maybe something scared him.
My dogs do that ... its just a weird dog thing ... try and get him to go outside or play with him ... give him treats he'll get better.