I have a 1998 Outback with 147,000 miles with the shakes. It shakes between 30 & 70mph, with the power on, back off the gas and it stops. I put the fuse in for AWD and drove it, no difference. The frequency is consistent with vehicle speed, not engine speed. Six months ago a mechanic on ask.com said it was a inner CV joint, so I replaced the right front axle. It helped some, but now it’s worse.
By now, it is very possible that the left axle’s CV joint is also bad.
On vehicles with high odometer mileage, it really makes sense to replace both axles at the same time.