Your local dealer will probably swear on a stack that you cannot, as the joint is not sold as a separate part. According to the archives, the joint is available as a separate part and can be installed without replacing the lower control arm. Tom Stangl reports the joint is available at NAPA from Beck-Arnley, will fit both front and rear (at least on 1Gs) and costs around $40.
There have been reports, however, that the aftermarket joints that are available are not OEM-type joints, but generic joints that include a grease fitting. Despite this, there are apparantly no practical differences between the OEM joints and the aftermarket joints. Both types are "sealed" (by necessity), but the factory doesn't provide grease fittings for their ball joints.
For thos who really want OEM-style joints, Paul Lyons has found them at Autozone. They are made by a vendor called Perfect Circle; a part number is not available at the moment.
Those who prefer the original sealed-boot type may be stuck with purchasing the lower control arm, or shopping the boneyards for used control arms.
The Last Word: Ben Lauterbach very helpfully provided the part number from Advanced Auto Parts. It is TRW part 10371 for a 1g and reportedly runs about $30. Sorry to 2G owners, I don't know of a part for you. [Thanks, Ben!]