Hi,
I've been having this problem with my 6.7 2008 Ram 3500 Dually (930,000 km/578,000 mi) for a few weeks now. Sometimes when I'm driving, it will take longer for the transmission to shift into 6th. If I manually shift to 4th and then to 6th, the engine will rev up and go to 5th and then 6th. Sometimes I just have to go faster than usual to get it to shift. If I accelerate real slow it usually shifts a lot earlier and by them time I'm at 100 km/h, it'll already be in 6th.

I had a transmission flush done not that long ago and the mechanic said the transmission was fine. Could it be some sort of sensor? The problem seems to be inconsistent. Sometimes it's really noticeable and sometimes the tranny shifts just fine. I read it won't go into 6th, unless the transmission fluid hits 55F. Could there be something wrong with the sensor, that measures the temperature?