Mystery(misery?) vibration, I am determined to find a solution.

[seatech30]

I almost didn't want to make this post because I know it probably wont go anywhere, but I have to give it another go, hopefully my new observations can trigger someone's intuition or memory.

I've been up and down google with all sorts of keywords, and picking up cookie crumbs. I have 2 yaris (2008 sedan automatic, 1NZ-FE) now with this vibration issue, and a 2015 Corolla that's very faintly developing the vibration too.

As per usual and so many posts, I have cleaned MAF, TB, new plugs, PCV, hearing test on fuel injectors, all coil resistances are close, same with fuel injectors, even swapped the exhaust rubber holders, oh and yes all 3 (4?) mounts. I've tried filling in the mounts with polyurethane, the dogbone almost rattled the car (much worse), so I put the other one back. The transmission mount, I only filled the "top" gap, but left the bottom one clear. I have played with all hoses and nothing. I checked grounds on harnesses, as some claimed a ground kit helped their vibration. All were matched except the TB metal housing showed a higher voltage offset by about 0.25V.

From my searches, I'm suspecting a bad alternator, axle shafts, or TB.

When the car is running, in N, it shows about 13.5V, then if put it in D, it drops 13.4, if I put the AC on, rear defrost and hold the windows up, it'll go as low as 12.2V. From this post it seems very likely that this could be the culprit, if anyone without a vibration issue could confirm their voltage doesn't drop this much, I'd greatly appreciate it. One from Toyotapartsdeal is $454, rock auto has reman for $170-200. Does anyone have any experience with the remans by any chance?

When I lift the car up and rotate the wheels forward and back, there's some give, but there is a lot of clickety clackety that seems to come from the transmission. I need to compare this to my older yaris, but I think it is normal. Perhaps I can post a video if that might help someone give me a clue? Cost seems to be around $700+ for both axles. From what I've read it's best to stay away from aftermarket ones. This would require me to do fluid change, which I have to do anyways, so that's probably closer to $800.

Finally, the TB, I don't have any ideas as to why this could be bad, just I've seen too many posts with it as a culprit. The RPMs dip below 650, 620 even 600 in D. When in N it stays within spec 700-750, and vibrations are not as pronounced. Cost $413.

There seems to be a definite correlation with engine load and or electrical current draw, as the radiator fan also causes the vibrations to worsen.

I've been breaking my head over this for a while now, so I apologise if I sound whiny or abrasive, just frustrated. I would really not want to spend $1500 to find out these didn't work.

Edit: after a long drive and mulling over this issue, I came to a realization that the 120Nm of force to tighten the back bottom mount basically "welds" the two pieces that go in the back/bottom. With that in mind, I figured it's probably best to lift the engine/trans as high as the other mounts allow prior to tightening this bolt. I gave that a shot and the vibration was definitely dampened. This would explain why the vibration seemed to worsen even though I'd only messed with the back/bottom mount. Just thought I'd note that for anyone who comes along this post in the future. Still need to figure out why the engine is causing the vibration, though. I used a phone with a "spectroscope" app using the accelerometer, and the engine is the highest source of vibration. I was able to rule out the mounts definitively by placing the phone before and after each mount, and noting that there is basically no vibration passing through the mount. In other words, vibrations oj the side touching the engine/trans, and no vibration on the opposite connecting end to the car frame.