The one I needed (and I'm pretty sure you need, because the other is for short bed) is the 15811 catback... BUT, read on...
I was informed by Magnaflow
and the documentation on their website before buying it that this is the correct bolt on solution for my truck (2000 4wd 3.6L Manual Xtra-cab). When I received it, it was about 2 ft too short!
I called Maganflow directly and spoke with a tech. He said it is indeed the correct catback. He said they get all their trucks from a dealership in California, spec it out and design their exhaust systems. I did some research and here's what I found:
My truck has ONE (1) catalytic converter, despite the Maganflow tech insisting it should have 2. Reason for this? California has stricter emission laws which requires 2 cats in-line instead of one (like mine has here in Canada). So when they took their measurements to design the catback for my model of truck, they only considered the California model. The catback I received was missing the exact length of an extra Cat because mine only has one.
I called Maganflow back and they didn't seem to care too much about my problem. Their solution was to tell me to add a length of pipe in between the cat and the new catback. They said they would consider updating their documentation, but I don't think they ever did.
I did eventually get it installed and i'm happy with the end result. It just pisses me off that you spend that much money for a "bolt on solution" and you end up having to mickey mouse it to work. I noticed a small gain in HP (it claims up to 8 increase) and it sounds awesome. I recommend, but be prepared to add an extra length of pipe and a couple clamps.