I've got two words for you James; torque converter... unfortunately, tranny removal is required to resolve this issue
It work something like this: Torque converter actual coupling speed only happens when the vehicle crest a hill <= your case James. Coupling is at that point between where the engine stops driving the wheels and the wheels start to drive the engine.
The rest of the time, there is up to about 10% slippage between the impeller and the turbine... slippage causes a cutting action on the tranny fluid, some tranny don't lock up the converter until high gear <= again, your case James.
Further more; in high gear, the gear ratio is 0.7 to 1, with this ratio and lower engine speed a standard converter would have more of a tendency to slip.
Though I'm hoping that my diagnosis is wrong... PM me if you have questions about the torque converter
