Can I omit 之 in 之后/之前/之内?
...前 and ...内 are the short version of 之前 and 之内 respectively.

They are completely interchangeable, ...前==之前， ...内==之内. 
三天前==三天之前，三天内==三天之内，第二次世界大战前==第二次世界大战之前

...后==之后 is the same thing. 两年后==两年之后 

The only difference is longer ones look more formal.