P0300 only when going downhill?

[Lsxtrucker]

I'm throwing a random multiple misfire only when or shortly after the truck is going downhill or stopped at a intersection where the front is lower than the rear. Within seconds it will misfire. 

 

Spark plugs are new oem, plug wires are all new oem and fuel filter was just replaced recently. 

 

Any help is appreciated

[sefiroxx]

Need a better scanner to show which cylinders are misfiring.

Single - plug, wire, coil, injector
Even/odd - harness, rail
Random - sensor (crank, knock, cam), maf, fuel pump

[Lsxtrucker]

11 hours ago, sefiroxx said:

Need a better scanner to show which cylinders are misfiring.

Single - plug, wire, coil, injector
Even/odd - harness, rail
Random - sensor (crank, knock, cam), maf, fuel pump

I took it to my buddy's and he scanned it with a snap on scanner. Its saying bad 02 sensor. 

 

I'm wondering which 02 sensors to replace as they aren't very cheap. I'm assuming I should replace the upstream set would that be correct? Would downstream cause a misfire as well?

 

Thanks