You shouldn't desolder anything.
From what I read when I looked up that library, the older GSM model used D4 and D5, (pin 4 & 5) for the software serial pins but the newer model was changed to use pins 2 & 3, so it is possible nothing is connected to D4 & D5. But it is never a good idea to change a shield pin unless you are absolutely sure it is not being used, and i'm not absolutely sure. So the better way to solve this problem is to move whatever you have on pin 2 right now to another pin.
Really all you need to do is set the pinMode for pins 4 or 5 as either INPUT or OUTPUT and see if that causes the GSM module to act strange. If it does, then you know you can't use those pins.