I know that vibrations have been discussed to death in many, many forums.

I think my vibration is either my driver's side rear wheel (lugnuts backed off so wheel was loose so it beat up on the studs - so the lug-holes in the wheel are pretty beat up) or the rear universal joint.

It had seemed in the past that the vibration was from 55-60+ and never varied, but now it seems like it happens from 45+ at steady-speed only, if I give gas or let off the vibration diminishes quite a bit (but doesn't completely stop).

Which sounds more likely? If I do both, the wife will start in so I'd really better be on the right track before I start.

Thanks, y'all.

because it sounds like you have a wheel on the left rear that is bad, sloppy at lug holes and the vibration seems to be from that area, I would be leaning that way, check your driveline, get the rear axle on jacks, put her in neutral spin driveline by hand and check u joints if you cant get any movement out of them up and down, or side to side, and they feel tight, there ok run the engine at idle in gear and watch the rear tires, good luck.

From what you saying, I think that I would start with a u-joint/ driveline concern. The other lug stud/wheel issue should be immediately taken care of, But only a joint or a shaft problem should be torque sensitive.
Rob

Check your axles. I had a vibration that I couldn't figure out, and my axles were worn and pitted on the bearing surface. New axles solved the problem.

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Mike Newby

69 Chevelle 355/TH200-4R
97 Grand Prix GTP
64 Chevy C10 Pickup
79 Suzuki GS550

Thanks for the ideas, y'all.

I tried a different rim and tire (a 20 year old tire with lots of dryrot so I didn't want to leave it on) and the vibration diminished but did not go away. It may be that I did not give it enough of a test, I was pretty nervous with that ancient tire.

Today it seemed like even at 25mph with very light throttle (just enough to maintain speed) I was getting the vibration, or anything above 55mph with any amount of throttle

Would front suspension slop or bad motor mount be possible causes for this too? How do you tell if your motor mount is going bad? My air filter has grown a dent, so I assume it must have contacted the hood at some point.

I'll be replacing the U-joint this weekend. Parts run should be happening tomorrow, so any additional advice is welcome.

Thanks again.
Derek

Just a followup,

I replaced the wheel expecting that the steady-throttle vibration would continue but the 55+ vibration would go away.

It did not.

As y'all suggested, my rear U-joint was shot - no needle bearings at all in one cup, and the other cup full of rust-dust.

Now all vibrations are gone. Thanks again.

Good deal. Glad it was an inespensive fix.

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Mike Newby

69 Chevelle 355/TH200-4R
97 Grand Prix GTP
64 Chevy C10 Pickup
79 Suzuki GS550