I would like my posts to pull my custom cropped thumbnail instead of creating its own thumbnail from Featured Image

If your theme is using the_post_thumbnail() to display the featured image, adding the following code in functions.php file will display the Thumbnail size of the image (as set on Settings -> Media Settings):

add_filter('post_thumbnail_size', 'my_thumbnail_size');
function my_thumbnail_size() {
    return 'thumbnail';
}

Or you can display the featured image in any width (X) or height (Y) you wish:

add_filter('post_thumbnail_size', 'my_thumbnail_size');
function my_thumbnail_size() {
    return array(X,Y);
}