Driveline shake in 4th, related to load

[The Lazy Destroyer]

'94 RMW
Car has a shake at approximately 75 mph. Feels/sounds exactly like when you hit the rumble grooves on the breakdown lines on the interstate, so it's pretty prominent. It's not exactly speed related, more so load related. So for example, cruising it may happen at 75 mph on a flat level. But may happen at 70-72 if going slightly uphill, or happen at 80 if going downhill.

So the speed it happens will vary to match the load if you are trying to keep a constant speed. Once it happens, if you immediately get completely off the gas, or get into the gas to increase the load on the driveline... it goes away immediately.

But it does not happen below 68-70 at all. Seems if you keep it above 80-85 it goes away but to be honest I don't drive that fast regularly to know for sure.
Feels like it comes from the center or maybe the rear of the vehicle but to be honest it's kinda hard to pinpoint exactly, and it's too strong to really want to stay in it to tell.

Previous owner said it happened with another set of wheels/tires so I do not think problem is there.

Any thoughts what this may be based on the description? Or anything that can be ruled out? I just got the car and will probably take it to my mechanic for a once over but curious what this may be.

I did a quick search and a couple threads had a very similar description but no resolution was posted

[Barbarossa]

My guess.
(five posts below your post)

Advantages of a larger aluminum driveshaft over a stock one?

Originally Posted by The Bolt-On Terror

Critical speed.
The lighter mass and larger diameter results in a higher critical speed (the rpm at which the shaft reaches its first bending mode natural frequency and turns into an oscillating jumprope).
Since you have 3.73 gears, at your cars top speed your driveshaft can reach an rpm exceeding that critical speed, and bad things can happen. Not WILL happen, but CAN happen. If everything is perfect on your car - ujoint angles, driveshaft balance, you might get lucky and never have a problem. The odds of that are probably less than 50% if you drive it at high speed.

Never exceed 110 mph, and you probably won't have an issue with the stock one.

[The Lazy Destroyer]

Stock 2.93's and I thought those issues were related to driveshaft speed and not load.

Am I misunderstanding the driveshaft issue when going to higher gears then? Is that similar when it's load related? I always thought DS issues like balance would be worse once it started if you sped up?

In the excerpt you quoted:

"Never exceed 110 mph, and you probably won't have an issue with the stock one"

This does not seem related to my issue. Every time it happens I can get it to go away if I just snap on the gas (change load) and it will go away before the car even gets a chance to change speed. Does not seem to happen at higher speeds since the load required to maintain it is higher than the load this problem happens at.

[Mr pogo]

I would check the usual suspects.
trans tail bushing play
condition of all u-joints
motor mounts
rear control arms condition and bushings

[eagle-co94]

Here's an added bit of info:

When towing an empty Uhaul flatbed trailer the vibration did not exist. After loading a roughly 2500 lb car on said trailer, there was also no vibration. It only happens when not towing.

Rear diff fluid is new (maybe 1000 miles) with the proper gasket and 2 bottles of GM additive.

[Mr pogo]

I shoulda added it (meant to but just woke up) but check driveline angles.
especially when a load changes the symptoms.

[DougDePew]

u-joints

[MillerTimeGS]

Tap the break pedal. See if it stops.


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[Navy Lifer]

Originally Posted by The Lazy Destroyer

'94 RMW
Car has a shake at approximately 75 mph. Feels/sounds exactly like when you hit the rumble grooves on the breakdown lines on the interstate, so it's pretty prominent. Once it happens, if you immediately get completely off the gas, or get into the gas to increase the load on the driveline... it goes away immediately.

Long shot, but check the harmonic balancer at the front of the engine.....