There are several possible causes for the symptoms you describe; further investigation by a mechanic will be needed to identify the exact cause of the problem.
First, if the vehicle seems :heavy" or hard to turn, you may have a problem with the power steering assist. If you sit in the vehicle and try turning the wheels with the engine not running, you will notice that the steering wheel is difficult to turn. Then, start the engine and try steering the wheels back and forth again while sitting still. The wheels should now turn much more easily; if so, the power steering is working; if not, there is a power steering system problem of some sort such as internal steering gear leakage or a failing pump.
You may have a tie rod end that is beginning to sieze at this age. Steering parts have ball and socket joints on the ends to allow the scomponents to change angle during turning. If one of these ball and socket type joints is not rotating freeely due to wear, internal corrosion, or lack of lubrication it will cause symptoms similiar to what you describe. The only way to know if this is occurring is to disconnect the steering components and manually check each one for ease of movement.
There are a couple of alignment angles which can cause difficulty steering if they are out of specification, but usually only in one direction (either into a turn or returning to center, depending on which adjustment and the direction it is out of spec). Since you describe difficulty boty steering into a corner and difficulty returning to center, I suspect that alignment issues are probably not the cause of your difficulty, but having the alignment rechecked by another shop may be worthwhile. If toe angle is too far positive, it will cause the vehicle to be difficult to turn into a corner, but the wheel will want to easily recenter itself. If toe angle is too far positive itwill also cause noticeable outside edge wear of the front tires after a short amount of time.
If caster angle is too far positive, the vehicle will also be difficult to turn when moving, but it will again want to return to straight after a turn easily.
It is also possible that one of your new ball joints may be defective, and not rotating easily. If you think of your front wheels as being similiar to hinges on a door, the ball joints aact as teh pins in the center of the hinge. If one is defective and not turning easily, the wheel will require abnormal force to move in either direction.
I would suggest taking the vehicle in to a different shop for a second opinion may be a good idea. A shop specializing in steering or suspension repairs would be a good place to try for a second opinion. They will likely also want to perform a salignment to check the critical angles I mentioned above.
If you do not know a good shop in your area, you can usually locate one using the shop locator link at www.iAtn.net