01 f150 rear diff Identification with no tag

I have an 01 4x4 that needs a limited slip rear end. I know it has 3.55 gears and 9.75 ring but there are SEVEN tag options. The problem is that there is no tag. How do I get the rite one. I have called the dealer and they can not help me. They said I would have to have the tag number since there are so many options. What do I do? The locking one in my truck now locks up randomly and needs to be changed out. PLEASE HELP