Rear axel is making noise can it be fix changing the grease in it?

if you have disc brakes on the rear then check the pad thickness
the pads have a scratch spring that when the pads are worn past safe thickness they make a scratchy / scream sound

It's not worth it to fix the axel, it is wise to replace the axels
by jacking up the car secure the car with jack stands, then remove the wheels, the big center nut of the axel and the lower bolts on the ball joint, then pry the lower control arm down and pull the hubb aside. Now u could remove the axel easily, if stuck take a hammer and tap it lightly, remove it, replace with the new but make sure both ends r lubed with grease.

Does the mustang have independent (IRS) axel or the solid 8.8 rear axel?

Sounds like the CV joint is bad. Take a look under the wheel and see if the rubber boot is torn and grease is coming out. Need to take to a repair shop.

Any Auto Parts store or even a Wallmart may have a WD 40. I would first try to spray this behind or right where you see the Axel of this Idle pulley.
Unfortunately it may be getting old and dried out.
If you have any grease or even - sounds funny - Vaseline - try smearing this on where the Axel meet's the round pulley.
Blessings,
A/V Guy

wheel bearing is bad...can't grease a sealed unit

My grandfather has a 97 Ford Crown Vic that used to do the same thing. He had the front end greased when he got an oil change and that solved the problem. So thats probably all you need.

Job is not terribly difficult, but instructions are too long to re-write them. Get yourself a haynes manual (about 20 bucks) and it will guide you through the job. If you need any help along the way, I'll be glad to help! (manual will help with lots of other stuff as well!!
If you have the setup I'm thinking of, make sure you buy a new diff cover gasket and gear lube along with axle bearing.
good luck

jack the wheel up and and sharp spin left and right should help you locate the problem. did you change both sets of the universal joints, if so did you really pack them well with the grease as sometimes if the grease is not well packed you can get a dry spot and this will heat up and become damaged.

change the rear brake shoes it worked on mine