1st, not all 4WDs are TRD...SO you dont HAVE to buy a TRD to get 4wd.
2nd, the TRD package for the Tundra is about the lamest package I have ever seen Toyota offer.
There is NOT,and NEVER has been an OEM locking diff available for the 1st Gen Tundras.
Every option that makes up the TRD package was offered as a separate option, EXCEPT for the expensive TRD sticker on the rear bedside.
Many Tundras that were not specifically TRD, DO have many of the TRD options on them....Depends what the original owners ordered, or accepted on trucks already equipped on the lots.
I would NOT pay extra STRICTLY for a 2006 TRD Tundra...If you find a good price on one that HAPPENS to be TRD, thats one thing...But to narrow your search down exclusively for a TRD equipped truck, I wouldnt waste the hundreds, possibly thousands of $$ a seller's gonna want for the added package.