Dave, thrust is the amount of allowable axial movement of the crankshaft. One of the mains (I forget which one on the 1UZ) serves as a thrust bearing, and when the axial movement exceeds a certain amount, it (and the rest) need to be replaced. Thrust increases through use of a very heavy clutch or other axial loading of the crankshaft.
It might be good for you to pick up one of the HP books on engine rebuilding and blueprinting. Most of these are written around domestic iron, but the principles still apply, especially for the block.