Go here: Ford 7.5 & 8.8 Inch Axle Tag & Door Codes
Look at your drivers door label and see what rear AXLE code you have and look it up on above list.
Assuming you have checked the fluid level in rear axle and it is full then:
If you have an L/S, Limited Slip, the friction additive that allows it to "slip" may need to be replaced, drain and replace the fluid with Ford L/S fluid.
If you have an OPEN axle then shims may be worn out