Just my thoughts.
It appears to rust seizure. First I remove the rust on the axle shaft. Here I would first try a coccus cloth but according to the image it appears to heavily rusted so it might take Muriatic Acid to quickly remove the rust. Now this stuff can produce toxic fumes so an organic vapor mask should be use along good ventilation. The collar will rust galled so that will be a pain to get loose. So getting collar heated to glowing red should allow you to use heavy pliers to twist off onto the cleaned shaft area. Basically you trying expand the collar so it can slip off the rusted area. Once removed it either needs to cleaned or replaced.
Caution do not get the Muriatic Acid on Aluminum as it will destroy it.
