How do I figure out what fitment my transmission is?

I am trying to find a used transmission for my 2008 Chevy Trailblazer. It's a 4.2L  
L6 Engine, with a 460Le transmission. However, every time I look at them online
it asks for fitment (I believe is what it called it) and then asks me to select
between ID SAD, SWD, TAD, and TWD.

I'm not sure what any of those things mean or how to figure out which will work
with my car. I tried asking my mechanic, but I'm not sure he understood what I
was asking, because all he said was that in his experience the 460Le fits most
Trailblazers and Envoys. Okay... cool. I really don't want to risk extra shipping
costs if I get the wrong transmission. I don't know if I'm just blind or what, but I
can't find anywhere that will tell me what the difference between these IDs are
or how yo find out what will fit my car.

So... please help?

Thanks in advanced and sorry for the long post.