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I'm no expert. But here are my observations from my pickup, which I put a manual TC lockup in.
If you are in OD (light off), and your speed drops down to around 40mph, the transmission will downshift to 3rd without engaging the (crap, can't remember exactly what it's called) brake clutch? Meaning, regardless of whether the TC is locked or not, the engine will drop to an idle unless you hit the OD off switch. In my pickup, with the TC lock on, if I'm in OD and the tranny shifts down to third, I only get the benefit of the lockup on the acceleration side, not the coast side. Dunno if this is the phenomenon you are experiencing or not, but in a stock transmission, the TC unlocking and downshifting to third are simultaneous. With OD on, there also is no compression braking action in the low gears, meaning it basically coasts to a stop with the engine at idle.
'03 F-250 7.3 4x4 ECLB
AIS Intake, Straight piped stock exhaust, Hypertech programmer, Amsoil Synthetics throughout