难得的一颗歪脖子灯杆站在了窗前: which translation is correct and is 了 needed?
B is correct.
Right, it's same.


Adding 了 makes sentence more natural, I don't why but
it would be very odd without it. 

歪脖子 is a common word describing a long thing, tree more often. 
a lamp pole is 歪脖子,that is definitely 难得的.

And there is a typo , 棵 instead of 颗.

Answer the comment:


脖子 represents the form of object. It must have a long part.
It is very common to add 了 after 在, and other verbs, 留在了, 排在了, 放在了, 打在了...


了 express 'past or perfect' tense.

他站在窗前.

他站在了窗前.

Both is correct. But for your sentence, it would be odd without 了. So there is still some difference.