old forced air blower

I have a 40+ year old forced air oil furnace and the belt-drive blower just started squeaking badly. It appears that the entire shaft that the blower blades spin on has slipped toward the back of the blower housing (away from the drive pulley, but the pulley is fine), allowing the blower to rub on the back of the housing. But I can't see why or how this could have happened or how to fix it. I just see that it has happened. I can push on the rear of the shaft and it will move a solid inch toward the front, and then start sliding back again once the blower is turned on. Making it more fun, there is no good way to access the blower. But if I knew what went wrong I might have a hope of fixing it.

old forced air blower