The carrier bearings always spin as long as the driveshaft or wheels are turning. Carrier bearings typically make noise under all road conditions as low as 10-20 MPH. To isolate the noise you can run the car on a lift or jack stands. Put your ear to it. Doing carrier bearings properly is not for the unmotivated. For D30/D35, if you are lucky you can take the old bearings off and press new ones on. However, variances in bearing thickness, etc. still may require measuring shims and reshimming. 8.25 requires tools for the side adjusters, setting backlash, etc. By the time you have to deal with all of this, it can be easier to get another good axle from the junk yard.