98 Avalon - Check Engine ON code P1780

Hi,
I need some help. Recently I bought a 98 Toyota Avalon (65K miles). Check engine light is ON. It was due to Oxygen sensor. So i fixed it for $400 from a mechanic. Immediate next day, Check Engine Light came ON again. This time it is code P1780 - which refers to Park/Neutral Position switch malfunction.

Now I want to know, how much this will cost now? what will happen if I don't fix it. While changing gears during starting, I could feel some jerk/jump. But it is smooth when driving at speeds. Any sugestions/help??? Thanks

Possibly loose or out of adjustment or defective. The cable to it also stretches over time and may need tightening.
Unfortunately, it may have been the cause of your o2 sensor code, because it signals the o2 sensors when the shift lever is in position D. If not adjusted properly then you may not be able to start your car without fishing around with the gear shift. It also affects your backup lights. The site below has info on it. It's the same as yours.
http://www.autozone.com/servlet/UiBroker?ForwardPage=/az/cds/en_us/0900823d/80/13/f4/46/0900823d8013f446.jsp#hd1-1-1#hd1-1-1 (http://www.autozone.com/servlet/UiBroker?ForwardPage=/az/cds/en_us/0900823d/80/13/f4/46/0900823d8013f446.jsp#hd1-1-1hd1-1-1)

Thank you so much csaxon. I am going to talk to mechanic with whatever inputs you had given. hopefully it does not cost much. Thanks again.