If it is the lower ball joint connecting the steering knuckle to the lower arm, good luck removing the cross-bolt without breaking the head off, especially if the area around that bolt is rusted, at all!
The ball joint will not come out of the knuckle until that bolt is fully-removed, as there is an indentation on one side of the OEM ball joint cup where the bolt runs (a replacement ball joint I installed has a ridge all around the perimeter as opposed to just on one side in which the bolt extends to hold the cup in the steering knuckle).
I replaced the right one and it took me three days to remove that bolt, even then, being as careful as I was, the half without threads broke off.
If that happens, you can drill the bolt out beginning with a small bit and moving to larger. Some people have drilled the bolt hole the next size larger and simply installed a hardened bolt through with a nut and lock washer on the other end as opposed to having the bolt threaded into the steering knuckle on one side of the split.