A locker is important off road because it splits the power delivered to the rear differential evenly between the rear wheels. This will allow you to continue forward progress even if one rear tire is lifted completely off of the ground. Virtually all stock "4X4's" have open differentials-- no locker or limited slip mechanism. In an off road situation where a wheel is lifted off of the ground, the vehicle can go no further. But driver skill is an important factor not to be overlooked. It is possible to simulate a limited slip by actually applying the brake to stop the spinning tire (the on in mid air) and transferring the power to the wheel left on the ground. This takes finesse and can be hard on the drivetrain as well. Not to mention the fact that you are now effectively driving with both feet in what often (al least here in Colorado) is an extremely precarious situation.
Better to just push a button and lock the the differential entirely. Now you can just worry about throttle modulation and steering inputs (and your trusted spotter--the guy telling you where to put your tires)
The advantage to a locker like the one in the TRD Tacoma is that before you hit pavement again you can unlock the diff and enjoy a smooth ride and effortlessly smooth turning. A locked diff on the street is a nightmare. It is extremely hard on the drivetrain, often breaking multiple components. It is also loud and bound to attract attention as your tires squeel and suspension jerks and drivetrain pops around the slightest turns. No to mention the diminished ability to keep the truck pointed in the desired direction.
You just have to go off road a few times to find that finding traction is usually the limiting factor to forward progress, followed by overall clearance and lastly by low, low gearing.
Welcome to a truly enjoyable passtime and congrats on picking a Toyota to do it in. The advantage of a Toyota is huge. You can spend your time and $$$$ modifying your truck for better off road performance instead of just paying for repairs (like your buddy with the Jeep.)