Ques ?????
What speed are your driving at? O/D stands for Overdrive. If you aren't going ( guesstimate ) over 50 mpg, I wouldn't have it in OD. Does it do that only at lower speeds or at all?
It does sound like classic "lugging the gear". This is one of those problems that you almost have to be there and experience it to determine as to whether the behavior is abnormal, or just "doin it's thang!"


