Most likely, it's an internal problem that'll require transmission
Technical service bulletin 03-40-015 gives details on diagnosing a 2-3
shift flare, but the things we're discussing here could cause this to
occur on nearly any shift.
The TSB indicates that the fluid temperature sensor should be checked and
that the adaptive values should be relearned prior to condemning the
transmission. You can see the TSB (and a host of other useful
information) at www.hmaservice.com. You'll need to register for a free
account, and the site requires Internet Explorer.