1978 D100 steering problem

My daily driver 1978 D100 has a steering issue. When you turn the steering wheel to the left, it turns only about half as far as it should. When to turn the wheel the the right it is fine. This truck has manual steering and brakes, slant six with 3sp on column. Suspension is not my strong point, but my guess is an issue with the pitmans arm. Any ideas? thanks.