Tranny problem?

[Mark Surks]

2004 5.4 Expedition 200K mi. When I am approaching a stop and take my foot off of the accelerator pedal, the revs drop to 1000 and when the speed gets below about 20mph(?) the rpm's drop to 600 which is where the engine idles while at a stop light. Occasionally, when I come to a complete stop, the engine remains at 1000 rpm and I have to heavily depress the brake to keep the car from lurching forward. Then a few seconds later the engine drops to 600 rpm and I can ease off the brake pedal to normal pressure. No check engine light so I haven't had it scanned for codes. Any idea what is going on here? If I show up at a trans shop are they going to have me rebuild the entire tranny? (I have never left a trans shop without anything less then a complete rebuild except when Ford paid to have it repaired under an extended warranty)