Many questions

[Lonestar10]

could also be the timing and metering actuators on the driver side of engine causing it to rock they are the actuators lined up above the spin on fuel filter on engine they are aligned this way Metering - Timing - Fuel Shut Off - Timing Metering

you could also make sure the overhead is set good if it is then use this tool https://www.amazon.com/3164001-Injector-...185&sr=8-1
to check if an injector is leaking combustion backwards this could also cause this issue.

rawze has a video or two on how to use it on the 871.

also the pipe on back of engine doesnt typically plug just the orange boost type boot on it will get hard and brittle and blow a hole in it.