I had Metalman do mine and he does nice work. He has a standard price per hinge and you should get that beforehand. He has a little writeup on the GMC site.
Usually the pin seizes in the strap and ends up pivoting in the frame. This is opposite to the design. If yours are like that the technique is to cut the pin at top and bottom between the strap and the frame, and then use moderate heat to drive the pin from the strap.
The newly made hold open springs seem to be defective and they snap as soon as the hing is moved. However, Chevy Duty claims to have bought a new batch which they think will work.
