The Power Steering - Diagnosis section of the VX CD-ROM suggests a few possible causes of:

Too much play in steering.

Possible Cause: Wheel bearings worn
Correction: Replace wheel bearings

Possible Cause: Loose steering gear or linkage
Correction: Retighten or repair

Possible Cause: Steering gear misadjustment
Correction: Inspect and adjust steering gear preload

Possible Cause: Worn or loose steering shaft universal joint
Correction: Retighten or replace steering shaft

Possible Cause: Worn steering linkage ball joints
Correction: Replace ball joints

Possible Cause: Worn upper or lower ball joints
Correction: Replace ball joints

There's also a section called Wandering or Poor Steering Stability.

Possible Cause: Mismatched or unevenly worn tires
Correction: Replace tires or inflate tires to proper pressures

Possible Cause: Loose steering linkage ball joints
Correction: Replace ball joints

Possible Cause: Faulty shock absorbers
Correction: Replace shock absorbers

Possible Cause: Loose stabilizer bar
Correction: Tighten or replace stabilizer bar or bushings

Possible Cause: Broken or sagging springs
Correction: Replace springs (pairs)

Possible Cause: Steering gear misadjustment
Correction: Check or adjust steering gear

Possible Cause: Improper wheel alignment
Correction: Adjust wheel alignment

A good alignment shop (possibly one that specializes in 4x4's) could probably at least check most of the items listed.