Vibrations in first gear but not neutral (W123)

[byronnash]

Over the weekend I replaced my engine mounts and shocks while replacing the oil cooler lines. It drove ok last night. On my way to work it started vibrating mostly in first gear. When I stop at a light, it stays smooth for a minute or so and then slowly starts to pick up a vibration. If I put it into neutral though, it stops. I don't see anything visibly loose or anything and the motor definitely is much smoother with the new mounts. Could it be related to the mounts if it stops shaking in neutral or does that sound like more of a transmission issue? I'm a little wary to drive it if it's got some grinding in the transmission or something. I don't know enough about these things to diagnose. Any tips are appreciated.