I seem to remember the lower intake manifold gasket being a problem on my old 1996 Windstar. I don't think they're much of an issue on the 1999+ models, but I could be wrong. I suppose if you're willing to take the time to replace them just for piece of mind, go for it. I didn't replace the LIM gaskets when I did the isolator bolt job on my '01, but I only have 72K on it.