How often I have change the oil from the diferencial

you check drive shaft and diferencial?

Once the differencial is making a sounds it mean that a major damage has been done to it. You can change the differencial fluid and visually inspect the gears to look for big broken metal pieces. If so you will have to replace it. Sonner then later the tires are gonna get locked and you're not gonna be able to move the trow without towing it.
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Check oil in diferencial and the bearings on the shaft/kardan.

TELL ME VECHICLE TYPE YEAR AND MODEL.

WHAT DO YOU NEED TO KNOW ABOUT IT

Sounds like vehicle speed sensor(VSS)

Your manual should help, if not check with the dealer's service department.

go to the bottom of the vehicle on the rear diff if your laying directly under the diff front being towards the motor back being towards the exhaust pipe tip, face the back of the vehicle next to the drive line on the diff usually the passenger side of the housing of differential there will be a plug if its not there its on the rear side plate.like i said im 90% sure its in the side of the housing anyway it takes either a big allen wrench or just a 1/2 ratchet with no socket to take off once you remove the plug get a smaller allen wrench,which is shaped like an L the shorter end dip it in the diff,if it comes out wet your good if not you wanna add carefull not to over fill i believe it takes 80/90 weight gear oil i havent been in a shop for a while but im all most positive its pretty simple the front if aplicable is the same scenerio

A Chilton book will take you thru that.

yes the speed sensor on the top of the diferencial controls the speedometer, milage, and the ABS breaking system. that light should be on as well, I had the same problem for about a year until I got myself a diagnostic computer, plugged it in and there I got all my codes.