gutted cat, o2 sensor, and center bearing questions

[gogg]

Alright I was wondering if the myth that if you gut your catalitic converter that your o2 sensor will get a false reading is true (on a 240sx if that matters). Also I was wondering, we had to take the driveshaft off of my car (for an engine swap) and my friend never marked the center bearing (aka the driveshaft support bearing) position, my car now shakes at 15-25 mph, I'm pretty sure this is the reason, I was wondering if it was possible to get my old bearing aligned or if I would have to buy a new one (new ones are perfectly circular right?)? Thanks.