A whine or howl from the axle area can be associated with gear wear, bearing wear, or an issue with gear adjustment. Low lubricant may contribute to damage, but topping up the fluid will not repair components that have already worn.
The timing of the noise provides useful clues. Note whether it appears during acceleration, steady cruising, or coasting. Tires and wheel bearings can also produce road-speed-related noise.
Contact Certified Transmission at 2112 West Broadway in Council Bluffs for an inspection rather than assuming the differential needs replacement based on sound alone.