The grinding sound is being caused by the gear on the starter and the teeth on the flywheel not being fully meshed when the starter is spinning, there are two possibilities, the most likely is that the starter solenoid is not kicking the gear on the starter motor out far enough or is not holding the gear out like it should until power is cut to the starter, the other possibility is that there are teeth broken on your flywheel and the starter motor gear is grinding there.
I beleive your solenoid is not keeping the gear against the flywheel, You need a new starter if it is warranty, otherwise you might try an aftermarket source for a solenoid, I dont think you can get just a solenoid from a toyota dealer.
Are you mechanically inclined? if you feel like trying a couple of things you could check for loose connections where the wires attach to the starter, or you can get a hammer and give the small cylinder on the starter a couple of good taps, it should look like two cylinders attached, one larger than the other, the smaller one is the solenoid, give it a couple of taps and try starting it again, you can also try taping it while someone else trys starting the truck if you feel safe under there and trust the other person not to do anything stupid.
I guess I feel like sharing my vast knowledge of starters with the world tonight, a starter is a small very high torque motor, when you turn the key the first thing that happens in the starter is that the solenoid energizes and moves a small gear that meshes with the teeth on the flywheel then the starter motor spins that gear which spins the flywheel, which turns the crankshaft, which raises and lowers the pistons and turns the timing belt that turns the camshaft that raises and lowers the valves and somewhere in there is a sensor or two that send info to the computer to get sparks flying
There are three common sounds a starter will make, if it is working normally you wont even be able to hear but it makes a whirring sound, if the solenoid is failing you will hear a grinding sound, and if the solenoid is completely dead you will hear a loud whirring sound that is the starter motor turning freely with the gear not having been moved against the flywheel.
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