Our rear shocks get seized up with gritty mud too SW (including all the front fork adjustments too)...you need to take it off and clean the area, putting WD in there will only create a "cutting compound" that will wear surfaces too...I used a MuttsNutz front guard on the rear and my shock/linkages,etc never get a spot of mud on them...here's a pic to show it (ignore the other massive mudguards lol, we ride all year round and have to walk into country pubs for a meal whilst we're out)...
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I've used the MuckyNutz ones on a few of our bikes, both front and back and they work well...I'm working on getting a supplier for sheet Impact-Proof Plastic so I can just cut my own shapes and fab my own motor covers for our BBS's too.