those are the upstream O2 sensor ports for late model 4.0L's. they're plugged, so you're good to go. just use the O2 port on the Walker downpipe suggested before.
if there was a gasket in place, it would have been a thin metal crush washer. those are also used on O2 sensors and spark plugs. the size is 18mm, and they are readily available at any local auto parts store.
look inside the exhaust flange. if the threaded plug are is caked with carbon build up, it's never been pulled out and you're fine. if it doesn't look like the rest of the manifold wall, then you might think about a new crush washer.