A little after having my side motor mounts replaced, my car began vibrating excessively when accelerating past around 3k rpms. When I went to put in my ES torque mount inserts, I found that my crossmember bolt was stripped on one side and had come out. So I am running with only one crossmember bolt. Is that what is causing the vibration? Or is it something else that could be doing it?

Sounds to me like you are right. When the car starts vibrating like that you need new motor mounts and/or inserts.

Ju§tin