As far as I know the ratios are different, and the entire axle assembly is also different.
A 2.5 with auto transmission is 4.875:1, the 2.7 XL-7 is I believe 5.125:1, the XL-7 uses a different axle, with a larger diff and some sort of CV joint on the drive shaft, rear springs I think are also larger. The front diff should be a cast iron unit that will swap over possibly requiring the third member mount to be swapped as well.