dan, if I am not mistaken the 3rd row got a LOT more popular in 03 04.
If you are looking at earlier trucks (or z71s) that may be why.
it was $250 to add the liftgate to an LS trim truck, and that was required for the PDP option so it didn't get really popular until 02 or 03.
All LTs were then liftgate standard + PDP standard.
Liftgate finally became platform standard in 05, but many loaded LS 03-04 I looked at had the 3rd row (or used to have it).
Z71s didn't get 3rd row option until 04.