RE: Mom takes kids for first swim of 2005!
Grab the driveline (with it in neutral and the vehicle properly supported/blocked so it won't roll) and see if you can wiggle it at the ends where the u-joints are. They should be VERY solid and not move side to side.
If the joints are greasable, grease them. If they are not, they could just need to be replaced.
Or it could be coming from the wheel/hub areas???