MONTE CARLO Driveline Vibration (I suspect)

[ed1948]

86SS, 90k miles, original, no accidents. It's always had vibration from 55 mph up since I've owned it 5 years now with the original Goodyears and now the road force balanced new Coopers. The vibration makes the dome lamp housing buzz.
Some observations and comments: rear u joint replaced 5 years ago, front tested good and tight initially but now(as installed) has a little looseness, The trans tail shaft has a little play and the seal is 2 years old. The pinion shaft has very little play. When a jack is placed under the nose of the diff housing there seems to be 1/2 inch of upward deflection(new adjustable upper links are on the menu as well as inspecting the lower track? bars bushings). The driveshaft has not yet been verified to be true.
Possible causes: (A) looseness at either end of the driveshaft causing vibration? (B) pinion angle not correct? (C) brake drum imbalance? (D) Bent or missing balance on driveshaft? or (E) too much awesome powa overwhelming the driveline from the 305...???

I've been busy with my move and home reno the last three summers and now, finally, plan to devote some attention to my Monte other than just driving it.

 

[303'505rollin]

Does this happen every where you travel? I swear I got a similar vibration issue but it's not always there it seams like it's messing with me lol. I feel like it's the road I travel on when I go to the next town cause when I travel in the other direction nothing and I even hit the highway at 75 and nothing, smooth with the cruise control on. I checked everything you checked and still the same result, if you figure it out let me know so I can try that also

 

[Mac442]

Are you positive the wheels were balanced? I had a vibration at about 55, tires were new and balanced at a local shop. So I replaced the u-joints, check driveshaft, checked axles, new shocks, pinion angle, chased it for a year. Pulled the wheels off and brought to another shop for just balancing, turns out they were horribly out of balance. Other shop obviously didn’t balance them even though they said they did.

 

[DRIVEN]

Also verify that your axles aren't bent. Unless you're the original owner, no telling if it got slid into a curb at some point. I discovered this on my car after replacing the axle. Passenger seat would wiggle at certain speeds.

 

[Kelowna]

I had the same problem with my 76 Cadillac until a old GM mechanic told me to replace the torque converter. Runs fine now. But check first U Joints, tire, and drivetrain

 

[79 USA 1]

This may be an apples to oranges comparison but years back we had a front brake rotor that was not machined on center that caused a similar problem on a 92 F150 2 wheel drive. We went through the u joints, re balance of the drive shaft, new tires and a wheel and tire swap. Same results, a vibration that started at about 50 mph and higher. We found the machining on one of the front hubs was off center with the casting and at speed the harmonics of the rotor running up and down off center gave us the vibration.

 

[SS_Malibu]

I'm gonna guess "C".
I had a similar issue with my '93 Roadmaster Wagon. Checked the wheel alignment, rebalanced my driveshaft and added new U-joints, checked for tail shaft and pinion play. After I sold it, I started thinking about how much counter weight was on the new brake drums I installed when I redid the brakes. I didn't think much of it at the time however, I wished I would have re-replaced those too just for piece of mind.