WARNING: This page is about a different car, the 2003 Nissan Altima and 2002 Nissan Altima. However, it is still accessible from the selected car via links, so may be relevant.
Check side gears, side gear thrust washer, pinion mate shaft, pinion mate gears, pinion mate thrust washers and differential case. If necessary, replace with a new one.
Fig 1: Locating Gear, Washer, Shaft And Differential Case Inspecting Areas