Top part of agitator loose

This problem is commonly caused by worn agitator dogs on the agitator cam assembly. The following link explains what the dogs are and how to replace them:

http://www.fixya.com/support/r823955-replacing_agitator_dogs

The part number is included in the link provided. You can purchase replacements at appliancepartspros.com for about $5. For a little more you can purchase the entire cam replacement kit, which includes the cam, the thrust bearing, and the dogs. If the model number you listed this under is correct, the part number for the kit is AP3094543 and sells for about $11.

NOTE: You can refer to the excellent exploded view parts diagrams the web site has to offer for assistance. Whirlpool also recommends the application of a little grease on the thrust washer. The part number for their recommended grease is AP3098553.

If you have any questions about this repair, please let me know. I hope this helps you.