Is there any realistic POD from 1990-2010, that the United States, Canada, Australia and New Zealand join to become one country or even just the United States and Canada?
The only "realistic" POD causing this, would be a last-minute nuclear war between the USA, USSR, and China in 1990-1991. The devastated Anglosphere unites together in an EU-like entity to rebuild. Decades later, this becomes a federalized country.
No. There is not any plausible ways unite whole Anglosphere. Canada is possible but you would need very early POD. Probably on early 19th century was last chance achieve that.