Are you sure the fan is vibrating or is something caught inside the fan itself?? My fan was making a horrible "vibration" sound that turned out to be a cellophane wrapper from a pack of gum. Anyway, here are some instructions:
I just removed the blower motor on my 2006 LJ a few days ago. It is a simple procedure, but a real PITA because you cant really see anything you are working on.
1. unplug the wire harness leading to the blower motor on the right hand side of the motor (as you are looking at it)
2. feel around the backside of the blower assembly at the 1 to 2 o clock position around the edge; you will feel a "tab" that must be bent down.
3. Bend the "tab" down (careful, it breaks easy) and simultaneously rotate the whole blower assembly counter clockwise.
4. the assembly will twist out and you may have to work it around to get it out.
Reverse the above to get it back in.
Once you do it, it will take you 5 mins next time.