'87 300TD AT Slow to Engage Drive

[jgl1]

1987 300 TD, 89k miles - when fully warmed, transmission is becoming increasingly slow to engage Drive, when shifted from Park or Neutral. AT fluid (Mobil 1 Dexron III) and filter are clean, fluid level is correct and selector rod bushings are intact. Once engaged, however, the transmission operates flawlessly. Is a catastrophic failure imminent? Should any specific maintenance be done at this point? A 700 mile trip is planned and would hate to break down in route.