You are absolutely correct that in Christ there is neither Jew nor Gentile. But I do not see where these Jews are in Christ.
The first time the New Covenant is mentioned is in Jeremiah 31:31. There is no mention of Gentiles in this passage. Only the house of Judah and the house of Israel are mentioned. The Northern Kingdom and the Southern Kingdom. This new covenant states that God will put, in Israel, his law in the inward parts, and write it in the hearts, and will be their God, and they shall be my people. And they shall teach no more every man his neighbour, and every man his brother, saying, Know the LORD: for they shall all know me, from the least of them unto the greatest of them, saith the LORD: for I will forgive their iniquity, and I will remember their sin no more.
This same passage is repeated in Hebrews 8:10-11.
To fully understand that this particular covenant pertains to Israel and not the Gentiles we read Romans 11:25-28.
Rom 11:25-28
For I would not, brethren, that ye should be ignorant of this mystery, lest ye should be wise in your own conceits; that blindness in part is happened to Israel, until the fulness of the Gentiles be come in. And so all Israel shall be saved: as it is written, There shall come out of Sion the Deliverer, and shall turn away ungodliness from Jacob: For this is my covenant unto them, when I shall take away their sins. As concerning the gospel, they are enemies for your sakes: but as touching the election, they are beloved for the fathers' sakes.
When it says, "For this is my covenant unto them" in Romans 11:27 the "them" is referring to the blinded, unsaved Israel. This is referring to a future time after the time of the Gentiles is fulfilled. At least that is what it says here in Romans 11.