If the box is there, it's in a utility easement. What that means to you is that even though you may own the property, utilities have rights to access it. Typically the title to your property will list any easements. Comcast may be able/willing to move the box, but not always. PS Call 811 before you build your fence, there may be other utilities in the easement.
I've asked a Comcast employee to help you. You should expect a reply in this thread.
Hi pinksouth06, I will gladly look into this for you. Please send me a private message with your account information (your name, address and account number).