The issue of an all-in-one lens issue aside, it looks like your AF is off, or both your lenses are front (or back, hard to tell) focussing... dunno wtf is happening in that first shot, but on the zoom lens, no part of the deer is in focus.
from the exif data
Code:
Exposure Time : 1/1600
F Number : 5.6
Focal Length : 250.0 mm
Exposure Program : Action (High speed)
ISO : 1600
Create Date : 2009:10:29 12:09:32.33
Is f5.6 wide open at that focal length for your lens? Could explain the blur if so, but why ISO 1600 at midday???
Personally, I don't see any problem changing lenses in the field - A good technique where one lens goes on the second one comes is good enough. Well, I've never had probs, and I much prefer that swapping between my 17-55 and 70-210 as I really love those optics, that I doubt exist in a full-range zoom.