Changed heater core, ABS light flashing when driving

I dove into the heater core project (I hope no one else needs to do this)... and after a couple days worth of work I got everything swapped out and back together. I filled the overflow up with coolant and then took it for a test drive. When I try to accelerate, the ESP indicator light (arrow circling caution symbol) begins flashing. Every single time I punch it this happens. I'll hit 2k rpm and the engine lingers here and dogs while the ESP/ABS begins to go grind and engage. My steering wheel was straight prior to the test drive but it is now cocked approximately 45 degrees while going straight down the road. I took the airbag off and recentered the steering wheel only to have it lose alignment on another test drive with the same results. Vehicle drove great prior to swapping the heater core.

Clearly there is something I missed... any ideas? Is there some kind of alignment or calibration I was supposed to do before driving the car?

Few details...

- The AC is still evacuated/empty.
- When I push the ABS button on the dash, I can punch it and seemingly drive the vehicle as it should. When breaking, it does seem to be rather "ABS-y" but I have never driven without ABS engaged so maybe this is normal?
- No check engine light and no codes on my handheld dummy checker

Any help or feedback would be appreciated. It's a long haul to a dealer and I don't have any trusted mechanics within a 150 mile radius...