Rachel WILCOXON was the daughter of William WILCOXON, Josiah's brother. A third WILCOXON sibling, Elinor, married Hugh CONN Jr., first cousin of Margaret CONN who married Josiah WILCOXON.
We know that a daughter of Thomas CONN of Frederick Co. MD was married to a John LEGG as of 1763, but the postings I've seen have John LEGG's wife as Anna NORCRAFT, who died in 1789. However, there's no date of marriage or source given. This Anna could have been a second wife, or there were two John LEGGs of the same approximate age both living in Frederick/Montgomery Co. MD when they met their wives.
The only clue to the identity of the John LEGG who married Thomas CONN's daughter is the fact that her inheritance was paid in Pennsylvania, not Maryland, money. Perhaps this John LEGG had moved from MD to PA by 1763.