Don't know if this is related or not, but I've noticed on my '96 that, on
the colder days of winter, it takes longer (e.g. has to be driven farther )
before the torque converter will lock up (that last little drop down on the
tach to around 2k rpm at highway speeds). My guess is that when it's really
cold out the temp sensor used to control tranny operation takes a little
longer to reach operating temp, and this tells the powertrain computer to
prevent torque lockup, I suppose to provide better "drivability" until
everything is warmed up. First couple times I noticed it my first thought
was that it wasn't going into OD, but pressing the OD lockout button would
cause a downshift and higher RPM, which tells me it actually was in OD. I
just figure the higher than normal RPM is really just normal torque
converter slip until the computer figured everything was warm enough to
allow TC lockup. Hope I'm right! :-)

Steve