Grinding when decelerating; I would be thinking U-joint or slip joint in the drive shaft. My Ford did this for a long time before I replaced the rear driveshaft. It would wear out rear u-joints in about 6 months because the slip joint in the drive shaft was shot. Any chance you have a SYE kit on your transfer case?

Before you buy anything put it up on jack stands and see if you can pin point the noise.

FYI if you change the gear ratio in the rear axle you have to also change the front to match it otherwise you can't use 4WD.

Sean

99 XJ Sport 2 Door, 4.0L, 5 speed, all stock. Daily Driver
79 F150 Standard Cab Short Box, 400ci, NP435, NP205, 35" SSRs