呀 and 啊 both seem to be used in the end of sentences for exclamation, and the only difference that I can think of is that the pronunciation in Putonghua (Mandarin) is different. In Cantonese, even the pronunciation is the same. Do 呀 and 啊 mean the same thing and can be used interchangeably or are their meaning different?
ABC's entry for 呀 says:
(replacing a (啊) when preceding word ends in a, e, i, o, or (y)u)
快来呀! Kuài lái ya! Come quickly!
呀 is a replacement for 啊 when it comes after a vowel.
This is also corroborated by《规范》's entry:
说话(huà)呀 | 车(chē)呀 | 你说(shuō)呀 | 起来(lái)呀 | 快去(qù)呀。
The simplistic way to remember it is: they are various forms of 啊 depending on (the shape of the mouth in) the ending sound of the previous syllable. There're also 哪 which is 啊 preceded by a final n-sound (人哪 Oh people!); 哇 (好哇；不够哇);啦 (preceded by 儿 sound in Northern-speak)；etc.
However, there is still a difference between them in some cases (need more context), sometimes conveying surprise, disbelief, amazement, etc.