Got a vibration that's confusing me. It feels like my typical exhaust vibration, but it's about twice as loud and pulsates at around 1hz. I can feel it in my foot and hear it as though the exhaust is pulsating. My go-to thought is driveshafts (which are both double-cardan) but that doesn't seem likely based on evidence:

This ONLY happens under the conditions of driving up a relatively steep freeway hill, at 75MPH or higher, under at least light throttle, in 4th gear (around 2800RPM). If I shift to 5th and even floor it, there's no vibration at all, even going up to 85MPH. Shift back to 4th and it's back.

So much has been going on I don't know exactly when this started. Could coincide with muffler changes, rear end ride-height adjustment (but currently pinion is within a degree of the shaft, should be fine), replacement of front shaft with double-cardan, rebuilding my rear double-cardan shaft, or a multitude of other unlikely things.


Doesn't seem like front shaft, because it happens in 2WD only when under some amount of throttle, the front shaft would do it regardless of throttle. But, it is a JK with the stock housing with good caster, which means I have about 5 degrees of separation between the pinion and the shaft angle (most people have absolutely no issue with this).

Doesn't seem like the rear shaft, because at light throttle in 4th gear it does the pulsating vibration, but if I go to 5th then at any throttle there's no pulsation.

Doesn't seem exhaust related, because that just seems unlikely, plus why would it only happen when going up hills. I can drive down a hill at 75-85MPH in 4th gear and give it throttle and there's no vibration (although, I've only tried this a few times).

Could the engine actually be going unstable? Also seems weird it'd be road slope angle dependent though.

Only thought I have which could maybe explain either drive shaft is that the hill causes the vehicle to squat a bit more, and the engine's vibrations at its operating RPM excites a resonance in the double cardan shaft, explaining why it both only happens in 4th gear and up steep hills. But this is just crazy speculation because I can't come up with anything else.

So, I'm at a loss for what could be going on... any thoughts? It's happened probably two dozen times over the last few weeks, but I can't directly trigger it, sometimes I'll go up the same hill at what seems like the same speed and it doesn't start. Whenever it happens, if I take my foot off the gas it instantly goes away, and I can slow to 70MPH then give it throttle and it's fine. I'll pull my front shaft and drive around for a week and see if it ever happens, other than that I'm not sure what to try (can't drive around at 75MPH without rear shaft for test due to front pinion angle).

Sounds like front driveshaft vibration. Remove the front shaft and test under the same conditions.

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Why would a front driveshaft vibration be at all throttle dependent? And even more strangely, RPM dependent? Does the engine actually shake the joint at resonance as I was thinking?

In 2WD, it shouldn't. EDIT: but it couldn't hurt to pull it to rule out

Maybe your rear pinon angle needs to be dropped down a degree or 2 so it aligns with driveshaft angle when it rotates up under higher input torque (just wild speculation - only a question, not an answer).

Ya, I'll pull it to check either way if I can't figure this out, but I really would be very surprised if it's the front shaft. Especially with 3.73 gears.

What's just super weird is that in 5th gear, at higher torque levels it doesn't do it (I can have it vibe in 4th gear with throttle to maintain speed, then go to 5th and floor it and accelerate up the hill but the vibe goes away when up shifting). I'll play around with rear pinion angle too, but again the evidence makes me really think it's not related to that.

* LOWER * ↗

In 5th gear, you are sending less torque to the rear drive shaft than in 4th (because less gear reduction).

That's what prompted my rear pinion angle theory, but

That can't be, if I'm accelerating up a hill there's more torque to the rear drive shaft than if I'm maintaining speed, it doesn't matter what gear I'm in. What changes is that in 4th gear there's lower engine torque because higher RPM.

If I were flooring it in both gears there'd be less torque in 5th, but in this case I'm lightly pressing the throttle in 4th with vibe (no acceleration), or flooring it in 5th (accelerating up the hill, more torque to axle) with no vibe.

OK, I missed the part about different amounts of throttle,
thought you had torque reduction backwards. Carry on.