Power steering fluid type

On my 1999 Ford Ranger 4x4, have on several occasions experienced violent shaking in the steering, after hitting bumps (just ordinary highway bumps, nothing unusual). A couple of times on the freeway at speeds in excess of 65 mph, the shaking has caused the vehicle to be nearly out of control. One incident caused me to swerve into another lane; luckily no other vehicle there. Ford service folks 'vacuumed the power steering' and it seemed to fix the problem. Less than a month later, the shaking is happening again. Fluids are fine and there doesn't seem to be any air getting into the power steering reservoir. Any help? Incidentally, this is not a new problem. I've had the truck since 2001 and it's always done this. Originally I rarely drove it on the freeway, just country roads and such. I've moved and drive much of the time on the freeway and is now my only vehicle. The problem is definitely worse at higher speeds.

I have looked at the power steering pump and all hoses and pump is not leaking. I think I have found the problem it is in the power steering cylinder. The ps fluid is leaking out of the drivers side

It is the rack and pinion I pulled the whole front end out and it got replaced along with several other parts thanks for the help