So, this isn't a current problem the way I typically drive, but I found myself going past 70mph, and my car started shuddering hard. Like something was kicking back and forth. The tachometer needle bounced around like crazy, a couple hundred RPM difference though I don't have a precise number. Didn't want to drive like this long enough to get one.

Accelerating past 75 seemed to start smoothing this out, but with a speed limit of 65 and not wanting to damage things further I decided not to push it too far in testing.

While I only had a few seconds of this, throttle response was normal. Got more power when I tried accelerating past this, power let up when I took my foot off the pedal. So I'm thinking it's not the engine.

This was on close to level ground with no major loads.

I've never had this problem at lower speeds. I have not tried getting to 70 in anything but Drive(it's an automatic), so I don't know if it's specific to 4th gear or not. It is a fairly abrupt change, not something that gradually builds as I accelerate.

The research I've done hasn't found anything that's a really precise match, but most of the similarish issues seem to trace to the transmission, often the torque converter. Does that make sense? Any tips on further diagnosis and/or predictions on how long I can let this go before it is a drivability problem at 65mph or lower?