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Mine loves about 30 mph. At 40 to 45 it's way too loud and shakes. Though when I have to drive like that it's ear plug time.

Maybe the drive line does need balancing.

For what it's worth, the gentleman at the driveline shop told me that any vibration at lower speeds is more than likely not an out of balance driveshaft. He says that because the driveshaft is turning at such a slower rate than engine rpm at low speeds, any out of balance issue would not be felt. More characteristically he said an out of balance shaft would be noticed at higher speeds and feel / sound more like a harmonic thing. He then said that the u-joints were the most probable culprit and that mine were bad and needed to be replaced. Now that I have new u-joints in the intermediate and rear shafts (front shaft removed for the time being), I'll take her out as soon as our winter weather warms up next week and see if in fact he was right. Will keep you posted...
Dennis

Today was a beautiful, sunny Texas afternoon so I fired Fiona up and went for a short drive. She now will do 40 mph without feeling like she's coming apart at the seams. However, 35 still seems to be the sweet spot! After replacing u-joints on the intermediate and rear shafts and then removing the front shaft, she drives SO much smoother!! Thanks, guys, for all your input. This site is perfectly synergistic, as it is so much better as a whole than any one of our knowledge areas!
Have a great week!
Dennis

Where's the "thumbs up" button?

LOL, great news.

Anybody replacing the rear end gear size for a higher top speed?

The reproduction 4.89:1 ratio gearsets are your only option today, pricey and hard to source.

Originally, they were an option on the WM300 and similar Power Wagons.

The truck will pick up a bit of road speed, limited by available horsepower and will still be slow on hills.

Offroad, they are more limited in performance in tight situations.

PoW

I had considered the gear swap a few years ago since my truck is AF and we all know they don't offroad, just putt around the airfield and parades.

But decided against it as mine was running as is. I'm a firm believer of don't fix it if it's not broken.

PoW wrote: ↑Fri Feb 11, 2022 3:22 am The reproduction 4.89:1 ratio gearsets are your only option today, pricey and hard to source.

Originally, they were an option on the WM300 and similar Power Wagons.

The truck will pick up a bit of road speed, limited by available horsepower and will still be slow on hills.

Offroad, they are more limited in performance in tight situations.

PoW

"Limited in tight situations" - is there some other limit besides the loss of torque due to the change in gearing? I'm sticking with stock in mine, but curious if the 4.89 gears introduce other limitations.

Simply put, they are great for a truck that never leaves the pavement.

PoW