Can I replace just one coilpack?

you can replace only 1 or 3 or 6 or 8 (no engine size in your question,3.2L or 4.2L),but yes you can just replace one. www.rockauto.com ,for best prices if you can wait,they ship it to you,or if you need now,see local parts store.

Absolutely you can replace just one - then another - then another - so forth and so on until 'ALL' coils have been replaced!