irengineer

There is no hard and fast rule that applies to every model year or even every engine / trans / axle variant. When the TCC locks up is controlled by the calibration in the TCM / PCM (depending on whether the trans controller is standalone or integrated with the engine controller - newer trucks are standalone). Generally they all have a typical pattern for apply release, but there's nothing saying they couldn't have been cal'd differently from one year to the next. If you see a 2-300 rpm drop after the 3-4 shift the TCC is applying.