Replaced upstream O2, should I do downstream?

After a slight vibration at idle on a 99 xj that sounded like a tank I finally bit the bullet and swapped out the manifold/ upstream O2.

Omg what a difference. It's like a new car.

I don't know how the fuel economy will change but should I do the down stream O2 as well?

The car is exempt from smog for me. I have no codes.

Some posters say it impacts performance, others it's only monitoring the CAT for emissions.

Thoughts?

FUEL SYSTEM
DOWNSTREAM HEATED OXYGEN SENSOR
The downstream heated oxygen sensor is located near the outlet end of the catalytic convertor (Fig. 6). The downstream heated oxygen sensor input is used to detect catalytic convertor deterioration. As the convertor deteriorates, the input from the downstream sensor begins to match the upstream sensor input except for a slight time delay. By comparing the downstream heated oxygen sensor input to the input from the upstream sensor, the PCM calculates catalytic convertor efficiency. When the catalytic converter efficiency drops below emission standards, the PCM stores a diagnostic trouble code and illuminates the Malfunction Indicator Lamp (MIL). For more information, refer to Group 25, Emission Control Systems.

I prefer to do both at the same time. The choice is yours.

I won't bother with the downstream unless you have a code. As agreen posted, it doesn't affect the fuel ratio and it's just there to monitor cat efficiency.

You all are awesome.Thank's guys.