As Davide Gualano has been told. This one
$("#myDiv").load("myScript.php?var=x&var2=y&var3=z")
use GET method for sending the request, and this one
$("#myDiv").load("myScript.php", {var:x, var2:y, var3:z})
use POST method for sending the request. But any limitation that is applied to each method (post/get) is applied to the alternative usages that has been mentioned in the question.
For example: url length limits the amount of sending data in GET method.