Sounds like the TCC solenoid in the trans is not working properly.

The purpose of the Transmission Converter Clutch (TCC) feature is to eliminate the power loss of the torque converter stage when the vehicle is in a cruise mode. The TCC System uses a solenoid-operated valve to couple the engine flywheel to the output shaft of the transmission through the torque converter. Lockup reduces slippage in the converter increasing fuel economy. For the converter clutch to apply, two conditions must be met:

Internal transmission fluid pressure must be correct.
The ECM must complete a ground circuit to energize the TCC solenoid that moves a check ball in the fluid line. This allows the converter clutch to apply, when hydraulic pressure is correct.
The TCC is very similar to the clutch in a manual transmission. When engaged, it makes a direct physical connection between the engine and transmission. Generally the TCC will engage at about 50 mph and disengage at about 45 mph.

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