my point about the throttle valve was in regards to the harsh downshift.
As for the vibration I agree with the driveshaft/u-joints. Check them first like I said before. A lot of posts on Mopar forums discuss possible causes for vibration in these cars. As for me, my Bro and I have 2 different setups both with similar vibrations. One is a 64 dart GT 360/904, 8 3/4 rear with 2.94 "highway" gears, headers dual exhaust with xpipe, 345hp/420ft/lb dyno at the rear wheels. the other is a 76 dart 2dr/hard top with a /6, 904, 8 1/4 rear with 3.23 "get this slug moving" gears, single exhaust probably 80 or so hp at the wheels. And the weird thing is the Vibes happen at the same vehicle speeds give or take. What I have noted that they do share is 2 door hard top design, 904 transmissions (one fresh and one OE 80k!), solid fans, neoprene bushing in the front, aftermarket replacement strut bushings (do not pop into hole like OE, starting to lean this way), unknown differential conditions (slight noise from both although the 8 3/4 is from a standard trans/HD truck- noise expected), some really poor body work from previous owners attempts to make them "look" good. So I have done a lot of testing and checking myself and no results yet. Hope to find a solution to share with everybody else in the same boat sooner than later. Start with the "free" testing and go from there. the better notes you take the more help the "Pro's" can give you at a lower cost. (being a auto tech i have experience with "unknown" problems that come in with no documentation about what has or hasn't been done). Hope this helps so we can all drive Vibration free Mopars one day!