95 jeep gc front diff noise

i have a 95 jeep gc and i have a high pitch whine commeing out of the front of the jeep it starts at 25 mph and gets louder when i apply the brakes to stop can anyone tell me what this might be i have had a mechanic look at it and he said it is my pinion bearings and it would cost me 350 to get it fixed

Is the noise constant noise? When pinion beaings go bad they will make noise when under loading (apply gas pedal) and/or while at a coast (when you release the gas pedal, you will hear this change there will be a brief moment when you release the gas pedal that there is no noise then the noise will pickup again when the vehicle starts to slow. If there is no change in sound between pressing the gas pedal and releasing it then I would suspect either the wheel bearings are bad or the differential side bearings are bad.