I presume that you are connecting onto UA, so you will process at the midfield FIS. So long as your inbound checked bag meets carry-on specifications (and totals), simply pull the checked bag tag off the checked piece after Customs and proceed through the TSA checkpoint. You will be dumped out into the sterile side of the C concourse and can proceed to your connecting gate.

If it's more convenient, just pull what you need out of the checked bag and then drop the bag back at recheck and claim in at JFK. Then proceed through checkpoint and on to gate.