What is the average cost to replace the rear differential? Not fluid, but part? on 2004 Dodge Dakota

I'm being told by one shop that I need to replace the rear differential (the part) in my truck. They want an insane amount of money for it and I think they are over-charging me. The truck makes a steady whining noise that does not change even while rolling in neutral. If it was the differential, wouldn't that noise change or subside?

Asked by for the 2004 Dodge Dakota