Any idea what could be wrong?
I doubt if anything you did to the rear brakes/axle is causing your ABS dash light. Not impossible, but not likely, IMHO.
BTW, on the front wheel, how do you "clean the ABS ring" just by removing the rotor? Isn't the tone ring embedded in the front wheel hub on a 2002?
My suspicion, depending on what exactly you mean by "cleaned the ABS ring" on the front left wheel, is that you somehow caused the WSS (Wheel Speed Sensor) to be interfered with or otherwise damaged its hardware/wiring, causing the ABS to disable itself and light the dash light (which is why you have "no issues braking" -- you probably have no ABS either).
There's really no need to guess, though. If you have a Bluetooth-capable Android smartphone or tablet, all you need is the free
demo version of FORScan and a cheap scantool like the popular "BAFX Products" Bluetooth scantool ($21 on Amazon currently). You'd be able to
read the ABS code(s) yourself and, assuming the WSS is acting up, read the actual value from the WSS as you rotate the raised wheel or drive. The free "FORScan Demo for Android" version cannot
reset any codes, but I wouldn't worry about that until after you've read them. And some codes will reset on their own anyway once the underlying problem has been fixed.
If you have an iOS device, there's also a free "FORScan Viewer for iOS" version which I suspect acts like the free Android demo version, but I've never run the former.
Even better (long-term) would be any old Windows (XP or higher) laptop and a wired (USB) scantool. The basic (non-demo) Windows version of FORScan is both totally free and more capable than the Android/iOS (demo and/or paid) versions.
However you decide to tackle this, good luck! And please keep us posted on whatever you find.