Vibration under load
 

Yesterday my car developed a vibration that roughly corresponds to engine load. It's most notable in fifth gear, moreso when going up a hill. If I let off the gas slightly so that the car starts to slow down the vibration goes away. In the lower gears I don't notice this vibration at all. I did notice it once while using engine braking in 3rd gear, but then I wasn't sure I was able to replicate it when I tried again later.

I do think my motor mounts need to be replaced, but from Clark's Garage I was under the impression that vibration from that should start to disappear around 1500 RPM or so whereas this vibration can occur much higher than that.

Any ideas what could be the cause and what diagnostic steps to take?

could be your tranny mount or torque tube bearings

Any idea how I would check for those problems?