let me go ahead and briefly explain why this is happening, this used to be a bit of a common problem a few years ago when more of this vintage trucks and 4 runners were around, I owned a 90 4 runner for a few years and had this some problem.
As the fluid heats up while driving it may get to the point where it starts to foam up, this foam and air above the fluid gets much hotter than the actual fluid, the temperature sensor is located a little above the fluid level and senses to temperature of this hot foam and the light comes on. We found that using a different type of ATF solved the problem permanently, I think it was Nissan atf which had different anti foaming agents, not sure if it's even still available but there are other fluids you can try using, most fluids a better these days anyway.
I would recommend using Valvoline Max-Life which is a multipurpose fluid that is compatible with even todays WS type fluids and can be used as a replacement. I would highly recommend removing all the old fluid using a trans flush device, the add a can of Lucas transmission treatment, hope this works for you, unless there is a problem with slipping of clutches or internal pressure loss this should work.
Also make sure the transmission oil cooler is intact and has not been removed by a previous owner and if necessary you could also add an aftermarket cooler, the idea is to keep the fluid temperature down to keep the fluid from foaming up.