Let's start with diagnosis. When you say it squeaks when you turn the wheel, do you mean when it's rolling or standing still? If it squeaks when it's rolling, then it could well be a universal joint, but if when standing still, it's more likely something else, perhaps a bad ball joint or tie rod end needing lubrication.
The universal joints are located inside the steering knuckles of the front axle. If you look at the front axle, you'll see that there is a point at each end which pivots as you turn the steering wheel. The outer axle tube coming from the front differential ends here, and the area between the upper and lower ball joints (on which the steering pivots) is open and there you can see the axle shaft, the part that rotates, and the universal joint itself, which looks similar to the universal joints on the driveshafts. If you get down there and study the whole assembly it will make sense. Park in a shady spot, preferably grassy, too, and lie on your back and spend some time getting to know your XJ from beneath. You can reach in and wiggle the u-joints, and see if they're loose. No loose is good loose here. You can also jack up a wheel, and with the steering all the way over, check for binding, squeaking or rattling. The joints should be smooth and without play.
Replacing them is a separate issue. Here's the "short course." To do this you need to remove the front wheel bearing assemblies, as well as the shafts. To start with, you'll need a 36 millimeter socket and a big wrench handle, to get the nut off the end of the axle (this is done with the wheel still on the ground). Then off comes the wheel, then the brake caliper and pads. Hang the caliper from a spring or something with a piece of wire, so it is not suspended by its rubber hose. You then must remove 3, 12-point, 13 mm. bolts on the back side of the knuckle, which hold the hub unit in. This done, you will remove the hub unit. Depending on age and rust, this can be a quick job or it can take hours. A search of the archives here may help with hints and stories. You may have to beat on the bolts from behind, or use a slide hammer, or other severe methods. Sometimes a lead hammer used obliquely on the hub flange can do it. If you use hammering force on anything but the bolts, you'll need to put the nut back on the axle shaft temporarily, to keep from pulling the hub unit apart by accident. If you get any movement at all, stop, and soak the crack with your favorite penetrating substance. Make sure you pull straight. It sometimes helps, once you have movement, to hammer it back in again, and alternate in and out until it's loose. Once the hub is off, you can slide the axle shaft out, and replace the joints. This is done by first removing the almost invisible retaining clips on the inside ends of the cups, then hammering or pressing them out. A ball joint press is a great timesaver here, as is a very big vise, but in a pinch you can do it with hammers and an assortment of wrench sockets. The joint is then pressed together, the clips installed, and if it's done right, you'll have a nice, smooth, tight joint. If you find that it binds a little after you've put it together, take a steel hammer and bang hard on the sides of the yokes (90 degrees from the direction the cups press in) a few times to settle it. You then clean the axle shaft very carefully (so as not to ruin the internal seal in the axle) and slip it carefully back in. Watch for crud and sand in the outer ends of the axle tube, so as not to push it in with the shaft. Replace the hub (clean the mating surface, and use anti seize and you will not have to hassle with it next time), hand tighten the axle nut, put the brakes back on, then the wheel, put it on the ground and then retorque the nut very very tight before replacing its cotter pin. Drum roll. Open beverage of choice and relax a moment before starting on other side...
