2004 Lincoln Navigator 2004 Navigator

2004 Navigator, 66K miles, just completed the 60K mile check up in May. During the last month, have noticed that engine feels like it is lugging(?) when going above 45-50 mph. Gears shift normally up through 4th, when driving in O/D at around 46-48mph, rpm drops to 1200 and vehicle feels like it is loose, hesitates, loses power, shakes a little (kind of hard to describe or attribute to a particular component). Doesn't matter if O/D is on or off, when the tach is in the 1200-1800 rpm range at a constant speed or accelerating, vehicle feels like it is slipping. If I accelerate above 2K rpm and keep it there (O/D on or off, doesn't matter) then I don't feel it. Engine runs well aside from this, don't hear any unusual noises (I owned the car since the beginning). I have an appointment with the dealer, but I would like to get an idea of what i might be looking at so they can't buffalo me.

Thanks!