My car shifts rough in first gear but at no other time. What could be the cause?

I have a 1998 Subaru Outback Legacy wagon that when engaging first gear (auto tranny) will hesitate slightly upon initial load; however, pulls fine in every other gear to include reverse. Could this be a drive axle, cv boot or am I looking at the start of transmission failure. Approximately 188,000 miles on the vehicle. Thank you