1995 Chevy Astro Loose Steering

The steering feels very loose in general. When I say loose steering I mean that sometimes the van changes direction without me moving the steering wheel and sometimes I have to move the steering wheel quite a ways before the van actually turns.

When I am traveling at highway speeds and I hit a dip in the road, the van will sometimes veer to the left. Anytime I hit a dip in the road at highway speeds, the steering feels loose.

I had all the shocks replaced recently and the response to dips in the road seems better but the steering is still very loose. My regular mechanic is at a loss to determine the exact cause and can only suggest that I replace nearly everything on the front end. Is there a way to determine exactly which part or parts is in need of replacement? What is the best way to repair this problem?