Hello Chris,If the kickdown is operating in lower gears, then i recommend to have the transmission adaptations reset. Possibly the transmission has learned to shift like this. Also it is a good idea to replace the transmission fluid and filter. Since the shifting depends on the engine torque, engine performance should be checked. If, for example, there is not enough boost pressure, fuel filter is clogged- it would cause insufficient torque and incorrect shifting pattern.The adaptations and engine check can be efficiently done with Star diagnosis computer. Some independent MB shops have this tool besides authorized dealers.
Since the transmission is shifting fine otherwise, the problem is not mechanical, but rather an electronics issue.