Jesse Bamba brought me samples from a mango which has stem galls (see image on left). I sliced open some galls but found no insects or mites living inside.
A quick Google turned up a photo by Dr. Scot Nelson of a mango tree in Hawaii with similar symptoms. Dr. Nelson suspects that the disease in his image is crown gall caused by Agrobacterium tumefaciens. Galls were given to Dr. Bob Schlub and Roger Brown for further diagnoses.