98-2004 Tacoma's with TRD Package had a rear locking diff with button on the dash. So if you are shopping, look for the button on the dash. People love to slap on Stickers like p nut said.
2005-2008 - You could buy a TRD Offroad truck or a TRD Sport. The "Offroad" package has a button on the dash for the Locker. The "sport" package came with a limited slip in the rear axle. Both versions got the upgraded shocks.
2009-2015 (I can't speak for 2016 and may be semi inaccurate with what I'm about to write, sorry) - For the 2009 model year, all trucks 2wd base up to 4X4 SR5 including "Sport" models got the limited slip. So any truck you get from 09 up, has a limited slip rear end. The only difference between a TRD Sport and a standard truck would be the upgraded shocks. In order to get a locker, you have to get a truck with TRD "Offroad" and that gives you the push button locker on the dash.
That is my level of knowledge. I have a 2006 with an open dif and a 2009 with the limited slip. The limited slip does work well. I haven't felt I needed a locker yet in the 09. The 06 actually sees more off road time and I could have used a locker a few times but still made it through with a bit more momentum. Not my favorite way of doing it but it is what it is...
Have fun shopping!!! Just look for the button on the dash as you shop. Most places take a close up picture of the dash buttons these days. Ignore the stickers...
Best,
Jorge