If you have an open 7.5 with 3:45s its possible to rebuild it with a 3.73 LS setup, but you can't just throw the new carrier in, you will need to change the pinion gear and setup the bearings and backlash and such.
Now, if you have a whole axle, go ahead, just remember that having different ratios up front and out back on a 4x4 will tear your 4x4 system to pieces in anything other than deep mud or deep snow.