Rear main seal is what the dealer was talking about.
The seal itself is usually pretty cheap - it's the labor that'll getcha. The transmission and all the other stuff has to come off to get to the seal. Going off what my 94 book time is (5.9 hours w/ manual trans or 6.9 hours w/ auto trans), and going off an $80 / hour shop rate, I come up with:
Manual trans - $472 + parts / tax / misc. items (fluids, etc).
Auto trans - $552 + parts / tax / misc. items (fluids, etc).
On a 2003 4.6L, it'll probably be more, so I'd plan on $600 - $650 minimum.
If you want a much better estimate, call the dealer and see if they can tell you what book time is to replace a rear main seal on your vehicle and what the labor rate is.
Book time X shop rate = estimate.