In some cars, there is a switch that disables the ABS in certain gear combinations. Check your owner's manual. That's why on some cars, there's a little light on the instrument cluster that lights up that looks like a skidding car... telling you that the ABS is not working in this mode.

the abs will work until the vehicle speed drops below a set speed like 5 mph as i was told.

ABS brakes don't care what gear you're in. They're brakes. Not transmissions.

sure, why not ? it works on 2WD