I've only seen one knock sensor that was bad in all the vehicles I've ever worked on. That was easy to see becasue it was physically broken.
I'd suggest having them scanned first. If you see excessive noise on the channel then I'd check their torque. A knock sensor'* sensitivity is controlled by the torque of it into the coolant passage. They are torqued to 14ft lbs.
On the rear, it'* possible. Pull all your talent and coolest tools out. Like I said, never had to do one in the car and only on that one motor. He was getting a ton of KR.
00 Regal GS: All kinds of cool stuff.