when i bought my xjs a few months back, it made a thunk when you went over a bump. the seller explained that he sprays the shocks with wd40 and the problem goes away for at least a couple months. i have had the same problem with my vans, and a few other cars,and the same fix has worked, at least for a while.
we use a lubricant called "tri-flow" which seems to last for years, in many applications (the from door lockset of the house we bought in 2007, for example, one treatment, still works fine)
would it be worth giving this a try, or should i just not be cheap and replace the shock absorber assembies?