Question about two noises possibly related to rear differential fluid

[Bigham]

Been repairing parts and eliminating noises along the way and i have two more to go. 1998 Eddie Bauer, 4.0L SOHC model with 202K miles.

Had the rear differential gasket replaced last year (i think they told me it was refilled with straight gear oil) and ever since then I have a high pitched whine coming from the rear while driving and accelerating. I have become used to it but I would like it to go away.

The second noise is when I take off from a stop and up to approx 40 mph there is a a vibration, grinding, crunching noise that almost feels like I have a flat spot on a tire. Tough to describe but the noise is consistent, but it also feels like there is an extra clunk/thump every time the tires make a rotation until I go fast enough and it goes away.

I rotated and replaced tires so that is out of the mix. Could it be a loose drive shaft?

I had a leak in my axle seal by the tire and lost some fluid. I think I need to top off the fluid. Do I need to add fluid with the friction modifier? Could both noises be coming from the rear diff because of the incorrect oil being used? Do I need a friction modifier?

 

[Bigham]

I called the shop to confirm and they said they added Valvoline 80/90 with the friction modifier. Looks like it may be a mechanical noise

 

[koda2000]

sounds like a bearing or worse, worn R&P gears.

 

[Bigham]

How costly is it to replace each one of those?

 

[tenny]

How costly is it to replace each one of those?

normally when they go inside the diff at the shop, they replace the ring gear,
and the bearings (pinion and axle)....and anything else that looks worn..

be prepared for a few hundred dollar+ bill when this is done.