The rear O2 sensor is actually a monitor.What it does is inform the PCM how the catalytic converter is doing. What the PCM does is it looks at the difference in the switch ratio of the front sensor and the rear sensor to see if the cat is working(burning excessive HC's). Normally this is around a 9 to 1 ratio. If it sees the ratios starting to match then it will set a cat efficiency code P0420 or P0430.
If you leave it unplugged it will not hurt anything except maybe a drop in fuel mileage and you will also not pass inspection if this is required where you are.
On the other hand it may have been disconnected because it was sitting a P0420 or P0430 code which would mean you either have a lazy front O2 sensor or a bad cat.
I am pretty sure you have the 5R55E 5 speed tranny in this model.
Not exactly sure which part you have there here is a pic of the entire tranny
If you are talking about no. 159 yes there is a gasket no 158 in pic basic no.7L173
The gasket on the other side of the plate(between tranny housing and separator plate no.149 in the pic) is part of the entire gasket which is no.148 in the pic.
However if you need this one the plate and gasket no.148 comes together you can not get these separate.
No.150 you can.