Time to change out rearends.

After removing the E Brake cables from the brakes and removing the driveshaft, cut the 7.25 U Bolts with a cut-off wheel as they are not wide enough to fit the axle housings of the 8.75. You'll want to buy new 3 in U Bolts. Get some help top lift the rear end out through one of the springs unless you're going to remove the springs as well.