The vibration shakes my seat, and spreads all over the car. But it's only occurs between 1750 and 2000 rpm, any higher or lower is smooth.

This started happening after I installed an OBX headers I got from ebay, it came with both the headers and and the pipe after it, without the cataltyic converter.

I still have the CEL on, it seems that it won't go away. I took off the K&N intake because it was easier just to see if it had any involvement, but nothing changed.

Any ideas? Am I stuck with this?

So here's am interesting update.

The CEL is off after I removed the intake. I thought it was on because of one of the O2 sensors. Huh.

The vibration feels less than before but is still definitely there between the same range 1750-2000 rpm.