Can an non ABS EL diff be used temporarily in an EL with ABS if ABS on the rear is left unplugged?

I take it you're talking about the whole rear axle ??
The ABS pulse rings sit on the axles, behind the stud hub, the sensors should bolt on as well - Powerwise, pull the ALB(ABS) fusible link, the brakes will perform as per a non ABS system.