Chevy 2500hd o2 sensor problem
 

Replaced both o2 sensors forward of the cat with Bosch units and truck runs worse then with the old original ones, will stall and backfire.

Fuel milage has been getting worse so I figured new sensors should help, after running for a while I checked codes and po137,po157 rear o2 sensor low voltage. I also tried the new sensors one at a time and same problem, put the old ones back in and runs fine, just with poor fuel consumption.

Do the rear sensors have to be replaced at the same time?

(warning:the following may or may not be correct)
Usually the front sensors get gummed up first.
The rears are located after the hot clean catalytic gases and should last longer.

Sometimes aftermarket sensors are not to speck or connectors are handmade and not proper.
O2 sensors actually create a small ~.5V fluctuating voltage for the computer.
If signal does not get through, the computer may drastically change long/short fuel curves for the worse.

When disconnecting battery or changing sensors, sometimes it takes a few days or more for everything to resettle.
"Long-term fuel curve."
The fuel/spark was set wrong before but now the new sensors are telling the computer the correct things.

Drive it a few days and see if it improves.