If it were my truck, my inclination would be to replace the radiator....especially if this is the original unit.... I mean it is 20 years old. Just because you can see coolant flowing doesn't mean it is flowing at the correct rate (this is one of the tests a radiator shop should perform on a suspect radiator). And as was suggested, your rad could be plugged in certain areas so that only a portion of it is flowing coolant. Try removing the thermostat and see if the truck still overheats....if it does or runs hot then you probably need a new rad.
Some other thoughts... the temp gauge.... is this an aftermarket installation or the orignal factory setup? Could be the sending unit has gone bad. Does your truck have factory A/C (whether it's working or not)? These engines are equipped with a fan clutch designed to "freewheel" under most conditions and engage when the air flow thru the rad reaches a certain temp. Again, if this is the orignal fan clutch, I'd replace it also.
Good luck and hope this helps...
mike
85S10