Andrew,
I had the same issues when I did my new shock install. It drove me crazy for almost two months.
Not to sound smug but I was sure I had the mechanical steps correct, gear depth & T/Q and even the reuse of the top bushings I marked them for position. The one thing I didn't do was use contact cleaner on all the plugs before doing the dielectric. I have since used it on the fronts and those codes stopped, the rears are a different story. I still get the odd code & it's 3 yrs down the road.
As usual this is just a fwiw.
Tom