By "differential", I assume you mean the rear axle. While what you describe is possible, there are other issues to consider. Like needing a longer brake hose, wiring, shocks, and possibly needing a longer custom driveshaft. There also may be issues with the rear sway bar (my wife has my Jimmy so I can't check to confirm) and other suspension components.
While it may give you about 3-4 inches, it will actually lower the front because of extra weight on the front springs. You need to raise the front, and if you're going to do that, you might as well just get a proper lift kit.