Get under your car. I bet the axle seal on the drivers side is leaking. I’d check both, but I’ve seen three cars all with the same shake at 38-low 40’s. There’s also a low hum while there’s load on it. Mean if you’re on the highway maintaining 70mph the hum is present, and if you left off the gas it stops. If you let off the gas the hum goes away. You should check out your axle. If you see the leak, I’m positive that’s your shakeSo why would a faulty axle cause that at only that speed? Also if it was , it would do it all the time....It only does it every so often at that speed....