Update custom field from other custom data
SO I was able to figure this out, turns out I wasn't able to reference the custom data directly from the activity object but had to use the api to grab the custom data related to that activity object.

function checkActivityMakeRelationship_civicrm_post($op, $objectName, $objectId, &amp;$objectRef) {
/* This function checks if a specific activity was created and if so makes a related relationship
*/

if(($op == 'create' || $op == 'edit') &amp;&amp; $objectName == 'Activity') {
    $params = array(
        'sequential' =&gt; 1,
        'id' =&gt; $objectId,
        'return' =&gt; "custom_50,custom_51",//replace with your specific custom
    );
    $activity = civicrm_api3('Activity', 'get', $params);
    $contact_a_id = (int)$activity[values][0]['custom_50_id'];//note the appended _id
    $contact_b_id = (int)$activity[values][0]['custom_51_id'];// appended _id
    $result = civicrm_api3('Relationship', 'create', array(
    'sequential' =&gt; 1,
    'contact_id_a' =&gt; $contact_a_id,
    'contact_id_b' =&gt; $contact_b_id,
    'relationship_type_id' =&gt; 11,//your specific relationship id
    ));
}


}