The problem actually started a little over 3 years ago.  I had previously swapped (from a 94 Caprice 9C1) into my car a 96 Impala SS rear axle with 3.08 gears, Metco extended length upper and lower control arms, Herb Adams front and rear sway bars, and steel brake lines.  Also swapped was a Dynotech MMC aluminum extended length driveshaft.  Not long after the swap was completed, I changed to 3.73 gears.  The day after the gear swap, I noticed a "buzz/vibration" that appeared to start right after reaching 75 mph.  Spoke to the folks that installed the gears and they said that the "high speed vibration" I was experiencing was probably coming from the driveshaft.  When travelling, I rarely go over 75 mph, so I ignored the problem until June of 2005.  I was going to attend ISSCA's Mountain Madness in Pennsylvania and participate in drag and road racing events.  Now I really needed to get the problem fixed.

I started out by taking the car to a driveshaft shop in Birmingham.  They checked the balance and found no problems.  I had them install new HD Spicer universal joints and check the balance again.  No problems.  Test drove the car and the "buzz/vibration" was still there.  Took the car to a transmission shop and had the transmission tailshaft bushing replaced.  Test drove the car and the "buzz/vibration" was still there.

I then took the car to AutoSport in Huntsville, AL.  They are rear axle specialists.  They checked pinion angles first, they were OK, and then removed the driveshaft.  While turning the pinion yoke, they discovered that there was a lot of drag, apparently from the pinion bearing.  To make a long story short, they wound up rebuilding the rear axle.  New HD pinion yoke and seal, new Eaton 400 lb. positraction unit, and all new bearings and stuff, including axle shaft bearings.  3.73 ring and pinion were in great shape and were re-installed.  Driveshaft re-installed.  Test drove the car and the "buzz/vibration" was still there!  It was still there with the transmission in NEUTRAL.

A lot of conversation on the Impala SS forum indicated that a number of folks had problems with Dynotech driveshafts.  Movement of cardboard inserts possibly causing vibration problems.

Robert Rodgers, a NASSA member in Huntsville, had the exact same driveshaft I had.  We worked out a deal to swap our driveshafts and see what happened.  You guessed it.  With his driveshaft in my car, there was no "buzz/vibration" all the way up to 110 mph.  With my driveshaft in his car (3.08 gears), he couldn't find any vibration until he kept the transmission in DRIVE and got up to around 85 mph.  Then the "buzz/vibration" began.  It was then apparent that the driveshaft was the problem.  I had to leave for PA in 2 days and "twisted" Robert's arm to borrow his driveshaft for a week or two.  Smooth as a baby's butt up to PA, and back, and during 2 days of racing.

A lot of folks swear by Denny's HD Aluminum Driveshafts, so I ordered the extended length version.  It arrived at the house on the Friday after I returned from PA.  Pictures below.  I installed it and tested it up to 110 mph.  No buzz or vibration!  :-)

So, what do I do with the Dynotech driveshaft?  After talking to one of their VP's several times, I shipped it to them for evaluation.  They checked the balance and called and told me that the universal joints had too much clearance, side to side, and that they would replace them.  They called me again and said that they checked the balance again and could find no problem related to the cardboard inserts, but that it was still out of balance and that they would correct it.  5 days later the driveshaft showed up at my door.  Sadly, they didn't call me back and tell me what they found and how it was corrected.

UPDATE:  In January of 2006,The Dynotech MMC extended length driveshaft was installed in my 1995 Impala SS, along with Metco extended length control arms, and the rear axle was rebuilt with the original stock 3.08 gears.  I've had the car up to 100 mph in DRIVE and haven't experienced any vibration.