I also would remove the zerks and try pushing grease through them first, I doubt that the king pins or the bushings are the problem, and since you grease it that often I doubt that the grease is hardened.

1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy