Great question, Kazu...Parts quality is important, and safety is our number one concern with front suspension and steering linkage. Parts like these steering knuckle ball joints on your beam axle 1997 Jeep ZJ Grand Cherokee are often difficult to access.
Here are the MOOG part numbers for the correct ball joints on your Grand Cherokee, they fit a variety of Jeep beam axle applications:
Product ID: MOOG-K3161T
Front Lower Ball Joint
Fits Left or Right Side
Product ID: MOOG-K7403
Front Lower Ball Joint - Adjustable
Fits Left or Right Side. Allows 1-3/4 Degree Camber and Caster Adjustment.
Product ID: MOOG-K3134T
Front Upper Ball Joint
Fits Left or Right Side
Note that the lower ball joints are available as "non-adjustable" and "adjustable". The adjustable type is for setting caster and camber when the beam axle or C-knuckles are out of alignment. If caster or camber (during wheel alignment) needs adjustment, adjustable ball joints are a sensible option.
The factory (OEM) ball joints are non-adjustable. If your front axle has been true enough to keep the tires from wearing erratically and the steering is safe, you can use the non-adjustable replacement joints. A caster, camber and toe-in wheel alignment is the test.
Aftermarket ball joint and steering linkage quality varies widely. The major brands like MOOG, Raybestos, TRW, McQuay-Norris, TRW, AC/Delco, NAPA and so forth each have "premium" parts lines. When it comes to steering linkage and ball joint parts, "premium" does mean a lot. The contrast in quality can be extreme between "service grade"/"standard" parts and premium grade. Brands like Raybestos offer superior engineering and upgrading in their premium grade parts.
If you plan to keep the Jeep ZJ Grand Cherokee for a couple hundred thousand miles, then buy only premium grade. If 80,000 to 100,000 miles of mixed highway and very light dirt road use is acceptable, or if the vehicle is approaching the end of its duty cycle, then the standard parts from a major brand would be acceptable. I emphasize a "major brand" like I have listed. There are many "knock-off" or unlabeled products available off-shore, and some of them are of terrible quality or very old engineering and design.
To answer your question directly, Kazu, the MOOG parts are likely equal to the OEM Mopar joints. This would certainly be true or even an upgrade if these parts are "premium grade". Verify the part numbers involved and check the MOOG official catalog for details. The link is to Federal-Mogul/MOOG site directly.
As for price, I would shop quality first. If necessary, go to the manufacturer's website and see how they represent the various quality lines. If the parts you have can be identified by a specific MOOG part number, go to the official MOOG website and see how MOOG describes the quality for your parts. The part numbers on these parts will help determine the quality grade.
It's worth noting that the adjustable lower ball joints can be valuable for Jeep beam axle vehicles and wheel alignment. These joints can compensate for factory beam axle errors or sagging that takes place over time or on trails like the Rubicon Trail! Factory specifications are on the margins in many cases with these beam axles. Oversized tires also take a toll.
For our '99 XJ Cherokee, I would likely step up for adjustable lower ball joints when replacing the joints, which I will do as an HD video how-to at some point. These joints require alignment gauges to set them up properly since they can be adjusted for a range of both caster and camber compensation. See my article at the magazine on Jeep beam axle alignment. I use the XJ Cherokee as my subject. (There is also an HD video on a Jeep TJ Wrangler alignment that I did as a how-to, which you can access with this link.)
Once you select the parts and quality that you want, price shopping at Rock Auto, Amazon, NAPA, eBay, or wherever, can follow. Always begin with the parts quality first. Check which MOOG parts you now have by their part numbers.
Thanks, again, Kazu, for bringing up the chassis parts quality issue. Members will be very helpful in assisting when you have questions...We look forward to your posts.