yes there is a plug under the floor plate in the main wire harness, in fact there are 2 plugs, one is for diagnostic checking and the other is a memory reset (clear) wire terminal.
As for how it works there should be a jumper connector to plug into it to make it show stored errors. If this is indeed a GM 4.3 the connector should be a standard 4 pin plug and usually you would jumper pins A and D i believe to make it flash codes. The figure it shows in the page i see it shows the connector already plugged into it and it says to remove the plug to make it redisplay past codes. (this is backwards from other make machines i've dealt with but knowing toyota they may have changed this method to suit thier designs.)
Generally with most GM equipt trucks, if there is an active code it will display by blinking the engine MIL light with the diagnostic plug open, key on, engine not running, and by plugging in a jumper it will trigger stored codes.

As for the memory clear terminal it should only be used when you want to clear memory codes.