while there is a possibility that you just have a leaky intake manifold gasket, it's most likely that you have a bad head gasket. the 3.8's are known to devolop head gasket leaks around the mileage that your van has.
mine lasted about 140k miles before a head gasket went bad. fixed it in my garage over the course of about 4 days. if you're mechanically inclined and have a torque wrench, then it is a messy but doable job. the gasket set runs you about $250. check out this thread to see what you're in for:
http://www.ford-forums.com/ford-wind...fix-today.html
like others have mentioned, you can run a compression test on all 6 cylinders to verify whether or not you have a head gasket leak. but the best advice i can give you is not to drive the van (or even start the engine) until it is fixed. after i did my head gaskets, i drained the oil after about a week of driving it to make sure all the coolant and emulsion was out of there, and have been doing oil changes at regular intervals ever since. with that said, apparently the bottom end of these engines is pretty solid and can handle a little bit of abuse. the engine should continue to last if you decide to fix it.
anyways, best of luck with the fix. if you have any questions about it, i've been there and i check this forum just about on a daily basis, so if there's any way i can help you then feel free to ask.
p.s.
have you had your transmission rebuilt yet?
seriously though, the transmissions in these vans have a pretty high failure rate too, and if you haven't had yours rebuilt yet then the engine may not be worth fixing. not trying to be a downer, but if your tranny hasn't failed on you yet, it's just a matter of time...
