Your truck is equipped with a Perkins 1004.4 engine.
It does sound like the tandem pump needs replacement. A very simple test to perform is, at a stop turn the steering wheel all the way one way and hold it there while trying to move slowly in either forward or reverse. If the truck jerks while driving then that in conjunction with your steering indicates the tandem pump is faulty and must be replaced.
The tandem pump is a "piggy back" pump and the smaller one provides pressure for the steering and boost pressure. You can buy just the small pump but it is basically the same price as buying the whole unit.