That's exactly what my 91 2wd was doing when my brushes were going out in my alternator, except it would flicker on and off. I could usually get the charge light to go off by revving it up real quick, but it'd eventually come back on till it came on for good a couple hours from home.
I caught it once pulling in the driveway, hooked up the load tester and it was sitting just below 12, rapped it up and it was charging over 13 volts - alternator not charging.
I got brushes from toyota (shouldn't cost but $10-20) and changed them pretty quickly. Hardest part was pulling the alternator off the engine!
