As to Utu, there is some doubt whether it represents a real pronunciation or not.

My own opinion is that it does, and that the underlying stem is _atu_, which in Babylonian has almost the same meaning as _bar_ or _baru_, viz., 'to see.' 'Utu' would thus again designate the sun as 'that which shines forth.' It will be recalled, that other instances have been noted of the same god appearing under different names.

The most natural explanation for this phenomenon is, that the variation corresponds to the different localities where the god was worshipped.

The identification would not be made until the union of the various Babylonian states had been achieved.
Such a union would be a potent factor in systematizing the pantheon.
When once it was recognized that the various names represented, in reality, one and the same deity, it would not be long before the name, peculiar to the place where the worship was most prominent, would set the others aside or reduce them to mere epithets.
It may well be that Shamash was the name given to the god at Sippar, whereas at Ur he may have been known as Utu.

Ur-Bau (of the first Ur dynasty) calls him Utu also, when speaking of the temple at Larsa, but it would be natural for the kings of Ur to call the sun-god of Larsa by the same name that he had in Ur.