The interesting thing is you're less likely to get an advantage from ABS in all but the worst conditions if you swapped to rear disk, since they are a lot less likely to suddenly lock.
Those old systems are pretty awful. Basically a system that doesn't have a pump (will have a motor) is a low pressure accumulator and a valve to open and close it. Not really worth doing.
I'd get a good aftermarket kit like the Dr Diff or a setup from a first gen grand cherokee to convert to rear disk and you'll never look back.
You also need not worry about c-clips with rear disk because the axles cannot fall out (the caliper will hold them in place).