I'm no expert by any means, but I have asked the same question. A pork shoulder comes from the front of the pig and is cut into two pieces, the butt and the picnic. The butt can be known as a "pork shoulder roast," "pork shoulder blade roast," "pork shoulder boston roast," etc. The picnic usually has the skin still on it and can be called picnic roast, picnic shoulder roast, etc.
The hams come from the back leg, so "picnic ham" I would have to ask the butcher. Usually that cut on the ham is called the "ham shank" or ham shank end." Just my 2 cents.