I went through the "replace" debate on here not too long ago, and in my case a transverse V6 so the rears are not the easiest to access.
The consensus here was leave them, but if I insisted to buy either Motorcraft or alternatively buy Densos, which were probably the OEM supplier.
In the end, I put new boots on my old plugs. The genuine Motorcraft ones were like $5 each I think, so not a big deal and hopefully takes care of any potential weakness in the old ones. RA actually popped up with a "manufacturer recommends replacement" or some phrasing like that when I put spark plugs in the cart.