Slight surge when braking in O/D

[Buck99]

I have a 99 4x4 w/5.4 auto. When I gradually slow down to around 20 mph, there is a slight surge and the rpm's jump about 100 rpm. It does not do this with the O/D locked out. Any of you with similar situations? It's not very annoying, but the truck is still under warranty and I would like to fix it while possible. Is there an adjustment the dealer can do to make it go away?

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[Granpa]

Is the 4 - 3 downshift happening then? Could check by cruising at 50MPH or so, to be sure its in top gear. Then coast down, watching tach and listening for coast-down downshifts (4 - 3, 3 - 2, etc.). Don't touch the gas, or else you'll unlock the torque converter and mess up the experiment.
I know on one of these downshifts, I can hear the engine speed up a bit, as road speed drives the engine for a moment after the downshift. I never paid much attention to it, knowing what it was. Can't say how many RPM it goes up. If I get a chance, I'll try to watch the tach and see.
Maybe others can comment on theirs.