Change the ball joint as soon as you can. It's not safe to drive like that. Specially if it has already rusted due to lack of lubrication. If it pops out while you are driving at high speed you will not be able to steer.
New ball joint is very cheap . You can just bang out the old one and press the new one in.
'88 Benz 420 SEL
'79 Porsche 911SC ('87 Carrera Engine)
'00 BMW 540 Sports Package
'99 Land Rover Discovery
86 300E - Manual (sold)
88 260E (sold)
84 944 (sold)