Power steering works hard at low speeds parking etc put then seems ok when you get going maybe rack and pinion

The first thing you need to do is find the leak, then once that is repaired if the problem persists you need to have the pump pressure tested and if that is ok replace the rack and pinion steering gear.

Jack the truck up and see if it's hard to turn . Now if you did nor flush power steering system the pump could have gone bad . Most power steering pumps carry a warranty on them wouldn't hurt to swap it out he can do the work yourself but it sounds like your rack and pinion is starting to fail

How new are your tires and when was the last time for a front end aliment?That might be it.

There are few help links showing exact accurate procedure to replace Rack and Pinion. Click the link below and follow the procedure:-----
Pinion Seal Replace on Dodge Dakota? http://howtobyme.blogspot.in/2012/04/pinion-seal-replace-on-dodge-dakota.html

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How to Replace a Rear Pinion Seal.
http://schematicsdiagram.blogspot.com/2011/07/how-to-replace-rear-pinion-seal.html
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How to replace Power steering rack and pinion gear on Chevy Silverado 2500 HD? http://technoanswers.blogspot.in/2012/02/how-to-replace-power-steering-rack-and.html

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How to replace Power Rack and Pinion Steering Gear on Acura Car Models? http://repairhelpcenter.blogspot.com/2011/11/how-to-replace-power-rack-and-pinion.html

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How to replace power rack and pinion steering gear on Toyota cruiser? http://repairhelpcenter.blogspot.in/2012/02/how-to-replace-power-rack-and-pinion.html

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How to replace Power Rack and Pinion Steering Gear on Acura Car Models? http://repairhelpcenter.blogspot.com/2011/11/how-to-replace-power-rack-and-pinion.html


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Front Drive Pinion Seal Removal on Ford Explorer? http://repairhelpcenter.blogspot.in/2012/03/front-drive-pinion-seal-removal-on-ford.html

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How to Replace Driveshaft, Pinion Flange & Seal?

http://repairhelpcenter.blogspot.in/2012/03/how-to-replace-driveshaft-pinion-flange.html

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How to Replace Power Rack & Pinion Steering Gear On 2007 Ford Taurus? http://repairhelpcenter.blogspot.in/2012/03/how-to-replace-power-rack-pinion.html


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How to Remove Power Rack & Pinion Steering Gear on Ford Windstar? http://repairhelpcenter.blogspot.in/2012/03/how-to-remove-power-rack-pinion.html

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Power Rack & Pinion Steering Gear replacing details for Buick Park Avenue and Pontiac Bonneville

http://repairhelpcenter.blogspot.in/2012/03/power-rack-pinion-steering-gear.html

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How to Replace Power Rack & Pinion Steering Gear on Mercedes Benz Truck? http://howtobyme.blogspot.in/2012/03/how-to-replace-power-rack-pinion.html


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How to replace Power Rack & Pinion Steering Gear on Toyota 4Runner? http://howtobyme.blogspot.in/2012/03/how-to-replace-power-rack-pinion_27.html

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Power Rack & Pinion Steering Gear Replacing on Toyota Land Cruiser & Sequoia? http://schematicsdiagram.blogspot.in/2012/03/power-rack-pinion-steering-gear.html

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Power Rack & Pinion Steering Gear Removal on Toyota RAV4? http://repairhelpcenter.blogspot.in/2012/03/power-rack-pinion-steering-gear-removal.html

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How to Replace Power Rack & Pinion Steering Gear on Toyota Sienna? http://technoanswers.blogspot.in/2012/03/how-to-replace-power-rack-pinion.html

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These details will help.
Thanks.

could be that your rack and pinion is going out try draining the power sterring fluid and make sure you know how much came out and then put in some fulid and to top it off put in some Seafoam trans tune and drive it around for about 30 to 50 minutes and that should fix the problem it beats buying a new rack and pinion their expensive

start with checking your tire pressure if thats ok check the belt for proper tension and fluid level and last that your idle speed is set correctly while the vehicle is idle in drive not idle in park hope this helps.

Your PS pump is fine.

This vehicle has:

Variable-assist power steering: A power steering system that uses valves and speed sensors to vary the amount of steering assist according to engine or road speed. At slow speeds more steering assist is delivered and steering the wheels is easier; necessary for parking, etc.. At higher speeds, steering assist is reduced and more steering effort is required to steer the car, giving the driver greater feel of the road. Also known as Speed-sensitive power steering.

Loss of current to the magnetic coils would cause a loss of power assist at low speed. Coil resistance can be checked with an ohmmeter, and should read about two ohms. An infinite (open) reading indicates a bad coil (requires replacing the rack since the coils are not serviceable). Checking for shorts between both sides of the coil assembly and rack housing is also recommended.

Best wishes

check for leakage, i think it is oil seal on rack and pinion, if you dont see the leak, open the rubber boots on rack end, fluid goes there

power steering pump is/may be going bad or the rack and Pinion unit could be bad.

If it is hard to turn it could be the fluid is low. If you can't turn the steering wheel go underneath the lower rack and pinion right were the steering shaft bolts to the rack and pinion there should be a round silver cap looks like a plug if that cap is missing the pinion bearings are broken and the rack and pinion are locked up replace your rack and pinion