Hi,
After you undo the 4 bolts holding the retainer,Use an axle puller,slider hammer to pull the axle. The grease means the housing seal or axle bearing is gone and seeps out between the housing and reatainer. The axle bearing wll be noisy or eneven when turned if bad instead of smooth,you can only feel this when the axle is pulled out.Normally the seals leaking are an indication of a bad bearing. If one side is bad,I'd replace both bearings and replace the housing seals anyway,cheap insurance.When the axles are out rotate the driveshaft and see if the pinion bearing feels smooth.
Perry.
