Can you use rampage while stunned?

Yes. Stun only stops a unit from contributing its Might to combat damage — Rampage's damage is a card effect, not combat damage, so a stunned unit is both a legal choice and will still deal its full Might.

Rule 423.1.b restricts a stunned unit's damage only "in the combat damage step." It doesn't prevent targeting, choosing, or damage from spells/abilities (same principle as Challenge — see FAQ 9251). So if your stunned unit is chosen by Rampage, it deals its Might to the enemy unit as normal, and if you paid the additional cost the friendly unit also gets the +2 [S] this turn.