I put a different size tire on a rear (235/60 vs 235/70 knobby) worked fine for a month but now I get intermittent ABS brake lightings. Happened twice this week. Any relation?

 

So your tires are different sizes? The vehicle could be sensing different wheel speeds if so, and turning on the light.

Also, driving with different sized tires on your drive axle is hard on your differential because each side is spinning at a different speed, so it's not recommended for any extended period.