2001 F250SD 4X4 steering problem

Steering jerks abruptly right/left and sometimes locks up when sitting at a stop sign.. Thought it was the power steering pump and replaced it. Still have the problem.

Could it be the steering box or should I look elsewhere? Thanks for the help.

List of parts replaced resently........

Ball joints----------------------two years ago apporx. 30,000 miles
CV joints-----------------------same
stabilzer shock-----------------two month ago
front hubs and lockers----------two months ago
anti-swaybar links--------------two months ago
power steering pump/hoses-----yesterday