Toyota Repair: 95 t100 makes whining noise at around 50 mph, toyota t100, metal fragments

Question
i have a 95 toyota t100 that makes a high pitched noise when you hit about 50 mph and it gets louder as you go faster. i took it to a repair shop and first they said it was the transmission. then after replacing transmission fluid and seeing that there was no metal fragments coming out they said it wasn't the transmission. so they looked in the gears in the axle(its a 2 wheel drive if that makes any difference) and found that there was no grease in it. they said that was the problem and said after putting grease in it the noise will go away after a few days. its been over a month and the noise is still there. what should i do next?

Answer
I assume that you are talking about the differential, it didn't have oil in it which probably caused damage to the differential bearings and gears, once the damage is done putting oil in it doesn't solve the problem as you now know because the noise is still there, so now the differential should be replaced.