Externally the V6 and V8 diffs for VT2-VZ are the same, even down to the doughnut flanges fitted fitted and are interchangeable.
The difference is inside the diff where the V8 runs a 8" (M80) crown wheel where as the V6 runs a 7 3/4" (M78) crown wheel with matching pinions.
You can tell the difference from the ratios
M78 runs 3.08, 3.45, 3.7 and 3.9, M80 runs 3.07, 3.46, 3.73 and 3.91.