I have a 2000 Toyota Tundra Limited 2 wheel drive with 294K miles. It has recently begun to shudder when shifting when under a slight load such as going up an incline or hill. This occurs when the trans downshifts out of overdrive or with overdrive off. The trans fluid and filter have not been changed since purchased new. Probably not smart, but is the fact. The fluid level is correct and the unit does not leak. Any ideas as to the problem? Thanks for any and all help.