My friend Rémy LeBeau has posted a nice list of vocabulary items specific to Persian of Afghanistan along with transcription and Iranian Persian equivalents (so if you don’t read Iranian Persian already, they won’t be of much use). They are taken from contemporary literature, but that doesn’t mean they’re all formal – in fact, they cross over all registers.
One thing I want to add to this selection – which looks more comprehensive than the most I’ve seen so far – is that the bulk of the differences between the two variants, is, I feel, a matter of mere preference for certain words which are known on both sides of the border. Ie. both Iranians and Afghans know that both X and Y mean the same thing, but Iranians are more likely to use the former, and Afghans the latter (while foreigners typically know only one word of such pairs, depending on which variety they have more contact with). The list which I link to focuses on those Afghan Persian words, the meaning of which would not be recognised by the average Iranian reader.