Roger Brown has covered this issue in Toyota Trails and on this bulletin board, apparently it is fairly common. Roger says "Those bolts tend to rust to the steel sleeve inside the spring bushing. You need to break the grip of the rust to get it out. If its on the driver's side, try hammering the end of the bolt, leaving the nut on the end. Another option to try is some heat from a torch to heat up the bolt. Or try a thin pry bar between the spring hanger and try to press down on the sleeve while prying against the hanger and bolt head. Often you can get enough pressure on the sleeve for the teeth on it to dig into the opposite side of the hanger and hold it in place so you can turn the bolt and break it free. Some PB Blaster or other penetrating oil might help if you can get it into the sleeve. If all else fails, a Sawzall or cutoff wheel will cut the bolt apart to get it out, then replace it with a 9/16" SAE or M14 bolts."