The abs is pretty easy to diagnose without codes. First if the light comes on after you start moving it is a wheel speed sensor. If it does not come on until after you are reaching higher speeds and I assume this is your problem, that is the diff swap. It will definitely cause the abs light. As far as fixing both the abs and the speedo due to the diff swap, the only way would be to repl. the speedo and abs module with ones calibrated for the rear end ratio. A car that came with this ratio stock. The speedo could possible be done with some aftermarket speedo driver. Something that could be calibrated. This could possibly work for the abs also, not sure.
They both use different sensors. The abs uses the one in the diff and the speedo uses the one in the transmission.