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 - Sunday Drives - by Cowboy. Summer was finally upon us here in Northern Maine. We have two types of industry up here, one being "Forestry" (Wood Products) and the other "Farming" (Potatoes). There is no shortage of farm tractors and equipment around here! I have been restoring old Farm Tractors for the past 6 years, and have found it easier and less expensive to hit all the auctions and purchase whole tractors for parts needed. My wife who works at a local school, and only has weekends and summers off, while on t
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