O/D band or servo?

[the_don]

so coming home from work i go to get on the freeway and about 45 or so i feel two sharp jerks from the truck and relised i had no overdrive, 1-3 shifts fine, 1-3 engine brakes fine. do i buy part for O/D band, or servo?

 

[BrooklynBay]

Check the band adjustment before you buy parts to determine if this is the issue. Did you check the modulator's vacuum line for leaking transmission fluid?