... and I have to say that I agree with those that are thinking about your transmission. I have hauled a 36' gooseneck that was loaded to near capacity with our 2nd generation CTD Ram -- across the country -- and never experienced anything like you described in your original post.

A good transmission man is a wonderful thing, so you might try to find someone who's well-versed in the 47RE and/or the 48RE. We had ours rebuilt a couple of years ago and the guy that built our "new" 47RE for us -- with a dose of 48RE innards -- was very good about letting me see the entire process.

I learned a lot by finding a good Dodge auto transmission man.

 

I have an 06 with 35" tires and at 55 your engine will be turning low 2,000 rpms, I'd have to go get my chart out, but I'd guess mine is 2300 or so. You will never see 3,000 rpms in 3rd gear unless one of two things is happening:

Your torque converter is slipping BAD.

Or your tachometer is shot. The tachs on the '06's are known to go bad and mine gives me crazy readings all the time (that's why I have the chart I made up when it was working ). My tach tells me my truck idles at zero and shifts into third at 4500 now. It's all over the place.

That said, in tow/haul the shift into overdrive will be delayed if you have high torque loads on the tranny right around 50 - 55 mph. It will shift out no matter what at around 65, but under load it will stay in third around 55.

 

If his tranny was slipping badly, wouldn't he get some extreme or at least HIGH tranny temps ?