The first thoughts are a broken axle or diff.
Second thoughts are a sheared drive key in the axles.
So off with both rear wheels.
If the square keys are in good condition and the slots are still nice & square then the tranny is suspect.
With the wheels off press the brake pedal and check the brake lever on the transmission is moving it's full travel.
Because the clutch & brake are linked, if the clutch fails then the brake should work and visa versa.
