1998 Mustang GT sloppy / delayed steering

Did they check the rack bushings? Have seen those deteriorate and cause that.

Tires.

Why I said tires? I assumed you had already had checked\tested the components of your suspension and steering. You mention new ball joints and wheel bearings which to me suggests you've done or had done the basics of the process to solve your problem which apparently hasn't solved your problem. Am I right in this? What you haven't addressed is if your suspension and steering is stock or if the tires you've put on are the recommended size? It's important information if what you're running isn't stock. Again, I assume everything is stock if nothing is said to suggest otherwise. To try and help with a posted question any answer has to start from some reference point to suggest anything useful. More than likely no one here is familiar with your car so assumptions have to be made. As an extreme example, I know of 4WD Mustangs with 40" tires but I don't think anyone would default to an assumption that this would apply to your car.

About tires, the age of your tires doesn't make any difference. Unfortunately a tire's "lateral tire deformation" is a spec that isn't required to be stated and outside of the realm of racing tires I don't think tire manufacturers are required to share the information of testing results that go in to lateral tire deformation ratings. This is about what is referred to as "sideslip." Lateral tire deformation is a vector measurement of linear, transient and saturation forces as tested against suspension deflection under various conditions. At one time I worked out of a place that had a test track that was often rented out for tire testing and learned that typically most tire companies all use the same list of vehicles for testing so the different tires from the different companies have tires with ratings that can be fairly compared with the ratings from other manufacturers.

All of this amounts to new tires really means nothing for the problem you describe if the tires you have don't match how your is set up. There is a lot more to tires than how long they last, the load they are rated for and the amount of predictable traction a tire offers under various weather conditions. If this was all there was to a tires specs, a $400 Bridgestone wouldn't have much on a generic $129 tire but we know that's not true.

What you're experiencing as described assuming everything is mechanically sound is an understeer lag in your steering due to sideslip. That's tires resisting a change in direction. They can't respond to the demand put on them by your suspension when you turn at speed. Another possibility although highly unlikely if they were mounted at a tire shop is you have directional tires that are on the wrong side of the car. This usually involves the same problem though with centering the tires coming out of a turn. They would resist any change in direction which you don't describe. An induced over and understeer similar to driving on ice.

Sorry for the late response but I wasn't sure how I should respond. Unless you're confident that everything in your front suspension is mechanically sound and properly adjusted consider having it rechecked at a good alignment shop. I'm suggesting no muffler shops that does alignments. Find a shop that can check your Ackerman Geometry which most alignment don't look at.

If this doesn't solve your problem some things to consider are:

1. Brake drag and proportioning\bias. Basically have your brakes checked. If the shop doesn't think it's necessary to test drive your car, go to another shop.

2. Rear differential, If you have any type of posi traction, used as a generic term, have it checked for delivering the proper inside\outside wheel geometry and roll rates when turning.

3. A chassis tune. There are many different levels of a chassis tune but you want to make sure everything is properly aligned and within spec.

If none of this works your last options are to put in a stiffer rear sway bar or decrease your front spring rate. Ultimately both achieve the same result. This is all about the relationship between your car's center of gravity to center of mass. This is too involved for a forum discussion but briefly, deceleration transfer weight from the rear to the front of the car. Braking when going forward does the same thing. This is typically a racing solution for different tracks and conditions. Tires have only so much traction and it's not shared with the different systems requiring use of traction. Transferring weight from the back to the front of a car requires more of the tires traction reducing what's available for steering. I don't see this being a problem with a fairly docile street car but it is a last resort.