I get a mean shudder in 3rd gear from maybe 2-4K. It started after I lowered my front end about an inch more with my coilovers.

I haven't got my alignment yet so I'm thinking it's either that, an axle going or a motor mount; it seems most like a motor mount to me though as I had the same problem in my old Accord.

How can I diagnose the problem?

 

Axle / CV joint going bad. You lowered it, so the angle the CV joint is on has been altered or increased. If it's bad or going bad, your change in the operational angle will amplify it.

 

I agree, but take a look at the motor mounts anyway. This is similar to offroad vehicles that do a suspension lift and then blow u-joints/drive shafts. When you change the angle of the driveshaft or in your case axle shaft it puts more stress on it so if it were already starting to go out you would begin to notice it much quicker. good luck!

 

I guess with lowering the axles are gonna be going regularly, as I changed both axles when I got my coilovers installed in November. The roads are really bad here so I could see how that would do it. Sucks