Taunton Vital Records to 1850, Vol. 2, Marriages p. 149: "Dean, Nehemiah of Raynham and Ann Williams of T[aunton] int. Dec 24, 1816, m. Mar 22 1817 (marriage date from records of First Congregational Church of Raynham)."
There is only one Nehemiah in Vol. 1, Births, p. 131. I don't think the timing is quite right for your Nehemiah though, who apparently is from Raynham but I'll quote it anyway: "Dean, Nehemiah, ch[ild of] Enos and Lydia July 4 1798." I don't think this Taunton Nehemiah is your line.
BUT.. here's the fun part.
Raynham Vital Records, p. 90: "Job Dean and Sarah Gushee both of Raynham. Nov. 8, 1777"
Raynham Vital Records, p. 4: "Job Dean son of Josiah Dean & Jane his wife was Born November ye 15 1751 in ye 11th hour at night."
also on p. 4: "Josiah Dean & Jean Washburn were Married August ye 18 1737 by ye Revd Daniel Perkins" (the records sometime refer to her as Jane, sometimes Jean)
on p. 26: "Josiah Dean, of Raynham, & Jane Washburn of Bridgewater were Joyned in marriage by the Revd Mr Daniel Perkins on the 18th Day of August Anno Domini 1737 & here follows the Names and Births of their Children Nehemiah Dean was Born December 30th 1738 Saterday Abigail Dean was Born Septemr [sic] 21 1740 Lords Day Mary Dean was Born August 20th 1743 Sater Day Josiah Dean was born March 6th 1748 Sun Day Job Dean was Born November 15th 1751 Fryday The above named Nehemiah Dean Deseasd February [?13]th 1740 aged about Eleven years."
So, my theory would be that Josiah Dean and Jane Washburn had Job Dean 15 Nov 1751. Job married Sara Gushee 8 Nov 1777 and had Nehemiah Washburn Dean (bet. 1778 and 1798, prob. c. 1792) and he married Ann Williams 22 Mar 1817.