Ok. No nothing wrong with that! Actually now that I think about my Excursion has the same issue. It has a weak (by design or bad programming) cruise control. It will drop a lot of speed before the cruise kicks in and by then it needs to downshift to pick up that speed again. So if it's doing fine In regular Drive then you will be OK. I don't know if you said what it is or not, but you probably have 3.42 gears which is why it doesn't do well in O/D with the trailer attached.
Thinking about it, if it goes fine when you touch the gas pedal when it's in cruise it may be as simple as a vacuum leak to the cruise servo, or the servo itself is weak. With no trailer the engine has enough vacuum to operate the cruise. With the trailer attached and being under a load vacuum will be less which may be why your having the issue only when the trailer is hooked up!
