I had this exact same problem. There is a nylon bushing inside the wheel 'hub' that has slid out of place. It acts as the bearing between the wheel rim and the axel.
Prop that side of the blower up and pull the wheel right off. (Mine had a star shapped clip I had to pry off with a screwdriver before I could get the wheel off.) The nylon tube may stay on the axel shaft or you may have to push it out of the wheel.
Put the bushing back in the wheel, aligning the holes and drop the pin in so they stay ligned up. Put the wheel back on the axel untill the pin hits. Pull the pin almost all the way out and put the wheel on. You want the pin through one half of the wheel and and one half of the bushing so the holes stay lined up or you'll be pulling the wheel off again.
The wheel slides on and off real easy. I did this on a Sears 536.887752.. There are no bearings or other pieces that could fall out. I see this as a design flaw and I leave the wheel locked all the time.
Good Luck