My Ford Windstar SEL has almost 100K. Tonight it started making a ‘clicking’ sound (from the left rear end?) but only at speeds of 40 and over. Any suggestions? - besides getting a new car! 
If alloy rims, check for cracking.
If steel rims and wheel covers check for damaged hubcap (cover) mounting clips causing the cover to flex.
40 mph and over? Check those anyway.
There is a known issue with the rear axle on Windstars. (Since this is a FWD vehicle, this part should probably be called a lateral support beam for the rear suspension, but most people do refer to it as the rear axle.)
It seems that sudden catastrophic failure can take place with this part, usually near one of the rear wheels. It can result in loss of control, among other things, so this is a possiblity that you should not ignore.
Unfortunately, Ford has not yet taken ownership of this problem, despite quite a few incidents over the past few years. If you have this problem, besides getting it repaired, you need to report it as a safety hazard to the NHTSA website. A sufficient number of complaints can result in an investigation. An investigation can result in a recall. A recall can enable you to be reimbursed for your repair costs, so save the repair invoice if you have this problem!
If you want an idea of what to look for, take a look at:
http://www.carcomplaints.com/Ford/Windstar/2000/suspension/rear_axle_failure.shtml#23
Hoefully, your Windstar does not have this problem, but I would like you to crawl underneath a.s.a.p. in order to inspect the rear axle for a vertical fracture. If you do find this, please log back in to let us know. Heck, even if you don’t find this problem, be sure to log back in, in case anyone has better ideas for you.
Good luck!
Miranda
Have you checked the rear axle?
Do you see any evidence of a vertical crack?