May I insist on paying off a judgment creditor in cash. I told the creditor in writing (Certified Letter) that the cash for the full amount to satisfy judgment was available for pick up. Am I required to do more than that?

Yes you have to deliver it. They don't have to come to you. You owe the money to them. You can pay them in pennies if you want, but you have to pay them.

But since paying in cash has less proff of payment it is the worst way to pay from a legal view point

It's available for pick up? Can't imagine why the creditor isn't cooperating.