Driveline shake in 4th, related to load
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