1986 Pontiac Fiero Huge vibration noise

I'm having a big vibration trouble when I press the gaz around 2500-2600 rpm and 3800-4000 the vibrations are loud from the interior but I don't know if people outside can hear it. And on the idle the entire carcshake, as if I have a hug v8 motor but it's a v6 ( 3.1reforged ), someone told me it could be a exhaust vibration but it is really loud so I don't think it can only be an exhaust vibration. And looks liek it's affecting my performance I have around 185 hp ( new cam and headers ) but when I press the gaz it's like if I had only 130-135. Thanks for helping me