If you have automatic locking hubs: These hubs automatically engage and disengage based on wheel speed. You don't manually unlock them. They are always ready for 4WD or 2WD operation as needed by the transfer case. There's no lever or button to engage or disengage them individually.
If (unlikely) you have manual locking hubs: These would have a lever or ring on each hub that you would rotate to engage or disengage them. Usually, a position indicating "free" (for 2WD) and "locked" (for 4WD). Check the outside of your front hubs for such a lever.
In short: For a 1998 Jeep Grand Cherokee Limited, you almost certainly *don't* unlock the hubs. Your focus should be on the transfer case selector (2WD, 4WD, N, etc.) to control 4-wheel drive engagement. If you're having trouble with 4WD engagement, the problem is likely not the hubs themselves but rather the transfer case, axle components, or something else in the 4WD system.
