2000 s 10 drivetrain

I have a 2000 s 10 with the 4 cyl manual tranny. When I depress the clutch to change gears the rear end of the truck feels like it is on ice (wiggles).
When the clutch is released again it feels the same. Does this sound like a tranny, rear end differential or axle problem. It began all at once and is becoming steadily worse.

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