First thing we ever did in the tire shop I used to work at when something came in with a vibration problem was to check the inside of the wheels for mud buildup.
If that's ok, assuming the tires are ok and properly balanced, then your thoughts of a u-joint or as mentioned the carrier bearing are the next(and most likely) places to look. A miss in the engine that only shows up under load could possibly feel that way too. Try holding the brakes with it in gear and giving it gas(powerstalling the engine) and see if anything shows up there under load).
Turning the converter on the flywheel should not have any effect on balance, and in fact some of 'em will only go on one way.
You might also look at the pulleys on the front of the engine as it's idling to make sure none are bent or possibly the bottom pulley or fan/waterpump pulley did not get fully seated when they put it back together.
One final thought, if somebody else did the work, is that possibly they could ruined the balancer when the pulled it off and stuck one off a 400 back on there in it's place, although this would most likely be much more noticeable than what you're talking about.
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Today's Featured Article - A Brief History of Tractors in Australia - by Bob Kavanagh. After Captain Cook's exploration of the east coast in 1770 the British Government decided to establish a penal colony in Australia. The first fleet arrived in 1788 and consisted mainly of convicts who were poorly equipped and new little of farming techniques. The colony remained far from self-supporting and it was not until the early 1800's that things started to improve. Free settlers started to arrive, they followed the explorers across the mountains and where land was suitable set up farms. T
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