I have a 2010 SH-AWD A/T TL, and I've been struggling to solve a vibration issue. To put it simply, the car starts to vibrate after about 65 mph, when accelerating. It starts to get scary wobbly at about 80 mph, and again only under acceleration. The moment I let up on the gas pedal, the vibrating stops. How hard I'm accelerating seems to matter more than the speed for how strong the wobble is, like if I'm accelerating hard to quickly pass a truck at 65 it will wobble a lot. It doesn't wobble at all if I go full-throttle from a standstill, until I get to about 60-65 mph.
The wobble is stronger when I'm sitting on the left side of the car (so, when I'm driving) and weaker on the right side (when I'm a passenger and having someone else drive it).
Something I discovered on the internet was that I could get a hint as to whether the problem is front or rear by whether the wobble is stronger in the steering wheel or in the seat, but it doesn't seem that clear cut. I think the wobble is felt in the seat, but I don't think it's particularly stronger or weaker in the steering wheel. Maybe that tip applies better to other cars with hydraulic power steering? It really feels like the entire car is shaking from front to back.

Multiple things have been attempted to fix this, and all of them have had zero effect on the wobble. Again to be clear, none of what I have done has made the wobble stronger or weaker as far as I can tell, and I'm pretty certain.
I have tried:
- re-balancing my tires
- re-balancing my tires again at a different shop
- SWAPPING all four wheels+tires from another TL that does not have the issue
So I'm pretty certain it has nothing to do with the wheels or tires!

After some research, it seems that because the wobble is only under acceleration, I should've been looking at components in the driveline, so then I tried:
- replacing the front-left cv axle with a brand new one (aftermarket, from o'reilly)
- replacing the front-right cv axle with a used one from another TL

The front-left cv was replaced because of an unrelated issue (it was clunking on sharp turns), and that resolved that unrelated issue.
The front-right cv was replaced trying to fix this problem, and I since I had access to a one from a (write-off) TL that I had access to I decided to save a buck.
The issue is still identical!
After replacing it with a new one, I tried grabbing the front-left axle and shaking it in every direction as hard as I can, there is *no* play in any direction.
The front-right axle has no play front-and-back (of the car, which is left-to-right for me when I'm sitting next to the wheel) or up-and-down, but it does have a tiny amount of play in-and-out, although as far as my research has told me this is not a problem because it is so little I can only feel it and can't see it, it's less than a millimeter if I had to guess.
It was at this point that I found out the front-right axle connects to another tiny axle called the "intermediate shaft" which has a bearing that holds it in place, and if either of those go bad then it could also be causing the wobble. The problem is that it's buried underneath some of the exhaust pipes from the engine, so that would have to come out to replace the intermediate shaft, making it much more of a pain and far less DIY for me than the axles I already replaced.

Maybe it's the propeller shaft?
I had a mechanic friend of a friend get my car up on a lift and inspect the driveshaft and front axles (including the intermediate shaft), and he said everything looked good. I tried convincing him to take the driveshaft out to really look at it, but he assured me that he was sure there's nothing wrong with it, because it would be obvious from inspecting it without taking it out.
Maybe I should get another opinion on that?

I'm even starting to suspect the rear CV axles. Is that possible? I haven't managed to find anyone else with this issue who found the problem was a rear axle, everyone I've seen said it was fixed after replacing the front axles. I've had a couple dumb ideas of trying to intentionally disable SH-AWD (I've had that happen when the vtec actuator went bad years ago, so I know if I unplug it I can make the car FWD for a bit...) to see if the problem changes, but I have concerns of going highway speeds without VSA...

After trying all that and not having any success, I don't want to keep throwing new parts at it without knowing for sure what the cause is.
Does anyone have any ideas for how I could figure out the cause of the issue, without just throwing more parts at the car? As far as I see, my remaining options to fix it are:
- intermediate shaft and its support bearing (which is a pain-in-the-ass to diy for me because of where it is)
- propeller shaft
- one of the rear cv axles
- something else I don't know about?

 

Take off the prop shaft and drive the car and see if you have the issue. If not, then it's prop shaft, rear diff or rear axles (or rear wheel bearings).