Well, after chasing this for six months with much anguish, I can now begin the mental recovery process, as I replaced the guide rod mounts (the big bushings at the foot wells) this morning, and I knew I found the problem when the bad mounts were removed, and I could feel play in the shaft/ball of the mounts. Thanks for all of your help, and kudos to some of you who correctly diagnosed the problem (I insisted that this couldn't be the problem since they were essentially new...18 months!....and were a reputable brand...Febi/Bilstein. Also, I swore I heard knocking from the wheels/hubs and not at the foot wells.)

Anyway, a few key points for the benefit of everyone on this site.

1. If you are going to replace shocks, certainly get Febi/Bilstein. HOWEVER, for all other suspension-related parts (ball joints, upper control arms, guide rod mounts), steer clear of Febi!! The Febi ball joints I installed in April '08 were crap, and the Febi guide rod mounts I installed in Nov. '06 were crap. Go with Lemfoerder or Meyle....I just put in Meyle guide rod mounts. I bought the Febi mounts here at Fastlane....nothing against Phil or against Fastlane...I just want to provide good information here....I will call Phil on Monday and discuss with him in person...He is very interested in this type of feedback, and he is a very helpful great guy, and Fastlane is still terrific (its just that Febi suspension parts suck!). Note that All Parts Express (also linked to this site) has Lemfoerder and Meyle...so I encourage all of you to go to this site as well as Fastlane.

2. The key symptoms of the bad guide rod mounts...and I mean not the major clunking of totally failed mounts, but the initial klunking of the mount that is beginning to fail and developed a little play, are.....1. Knocking at low speeds..(10 to 25 mph) when driving on "washboard" road surface. At higher speeds the knocking does not happen because the guide rod mount is pre-loaded with the suspension pushed back by road friction/wind resistance, and 2. Knocking goes away with braking...same reason, pre-loading the guide rod mount. Note that these very specific symptoms were reported by others on this site...I had done a very detailed search of the archives, sorting through the comments that were unclear or irrelevant to my situation, and there were about a half dozen individuals who reported these specific symptoms of early guide rod mount failure...thanks to those individuals.

3) When guide rod mounts have totally failed, with lots of play, you can detect a major klunk by repeatedly applying and letting off the brakes...this is the standard diagnosis of failed guide rod mounts. But when there is only a little play like mine had, there is not enough play to hear knocking by applying and letting off the brakes. But when the wheel is bouncing up and down when driving over washboard rod surface at low speeds, the guide rod mount is not pre-loaded, and the rapid knocking is very pronounced!

Final note....I did change the guide rod mounts without compressing the main springs...pretty simple....I did follow advice of others at this site and used a come along....Loosen the lower control arm eccentric bolts, upper shock connection (don't disconnect these, just loosen!), and guide rod attachment at the LCA (this last thing gives you enough play to move the guide rod around to line it up with the new guide rod. Quite a bit of force is required to pull the suspension forward to provide clearance to remove the guide rod mount, so you want to make sure you don't pull your car off the jack stands....I parked my car with front bumper up against a tree and attached the come along to my wife's car...to the towing eyelet. Very simple...took about 3 hours to do both guide rod mounts.


Again, thanks to all for your help,

Mark

P.S. The front suspension is now completely rebuilt, and is so tight it drives like a Ferrari...well not quite that good! I am scheduled for an alignment Monday, and will put new snows on Monday night...I will be driving to New Jersey and back this next Wednesday (12 hrs. round trip) and will very much enjoy my ride!!

__________________
1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)