My 1997 manual 2.5l outback has some shaking features. Whenever I am driving in about 3rd or 4th gear and between 3000 to 4000 rpms it shakes a bit. It's not all the time but some times it's bad. My motor mounts are needing replacing so I know some shaking is from that. But i know not all is that. What else do I need to do?

Take a look at the driveshaft U joints.

Sound I look for anything particular?

Slop/excessive play.

Stiffness rough movement. You'd have to pull it to do it well.

I just went through this with my 01 Outback. I was hearing a roar at 50 MPH and the seat vibrated. Rear u-joint had 1/4 inch of slop. Be prepared to get your wallet out if you find a bad u-joint as they are not replaceable. You will have to buy the whole driveshaft. I looked on line and called autozone. I was quoted between $450 and $550 for a remanufactured driveshaft. I went to a local slavage yard and found a good used one for $135. Problem solved.

Edited October 16, 201411 yr by 01 Outback

There could be a business that rebuilds them in your area.  There is one in my area..