Quote Originally Posted by tbplus10 View Post
Check the rear brakes before you open up the diff, not unheard of for a set of shoes to break loose and cause this type issue.
UPDATE:
It is fixed! Should have done this first and saved my friend's money from spending on rear diff gear oil and gasket. Came to find out there was a bolt that broke loose in the rear park brake that was causing the rear wheels to lock up once in a while. Sometimes, it will drive fine and just lock up for no reason.