300E, 4matic, 1991, 131K miles. Transmission will not shift into 4th and sometimes it won't shift into 3rd. If you place selector into 2nd and return it to 4th before starting off, it consistently shifts from 1st to 2nd. Neutral and reverse can be selected and work appropriately. I've checked fluid level and vacuum connection. Fluid had a normal color and smell. I was going to drop pan next and check for metal. What are the probable causes. What should I try or check next?