if opp play Rampage and didnt pay the additional cost, can I use rebuttal to make a new choice and pay the additional cost for the card?

No — Rebuttal cannot let you pay Rampage's optional additional cost.

"Make new choices" effects only let you remake finalization choices: locations, modes, destinations, and targets. Choices made "as you play this" — including Optional Additional Costs like Rampage's [C] — are explicitly excluded (Rule 752.2). The spell was already played and its costs already paid, and any cost that would be incurred by remade choices is ignored (Rule 755). Rampage will resolve under your control, but since [C] was never paid, the +2 Shield clause won't execute.