I decided to try taking apart the shock absorbers, which have been sitting in a box for the last couple of years. I have read that you usually have to get medieval on these before they will come apart.
The first step was to put them on the barbecue grill at full heat for an hour and a half. For a while there was lots of foul-smelling smoke coming from it. Eventually all the crud (inside and out) got burned up. Then I dropped the hot shocks into a bucket water, and the shock of cooling hopefully loosened the screwed-together joints.
Then I was able to easily remove the outer reservoir covers with a chain wrench, revealing large amounts of slimy, sooty crud. With a jumbo-size adjustable wrench I was able to remove the inner covers of two of them, revealing the working parts (and more crud). Once that was cleaned up, those two shocks appeared to be in amazingly good shape, and probably just need new seals.
I could not get the inner covers off the other two shocks, but their shafts seem pretty loose, so I think they are worn out anyway. But even so, having 50% of the shocks being rebuildable is better than I expected. Later this week I’ll try repeating the barbecue treatment on those two.