Would water in the gear oil in the rearend make it slip
Hi there. Water in the rear end differential gear oil, would not make it slip. It could make a "limited slip" differential slip, contaminating the clutch packs in the carrier, resulting in only one wheel doing the work.
Regardless, the differential needs to be serviced to remove the water, as soon as possible. Not removing the water will result in catastrophic and expensive repairs.
If you're unable to do so yourself, I would recommend having your vehicle's differential service be performed by a certified technician, such as one from YourMechanic.
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