I doubt it is the U joint. If it is bad enough to clunk, it would vibrate like hell, when under load.
One possibility is a worn or broken exhaust bracket allowing the muffer (or exhaust) to hit something.
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BroncoJoe19
2006 Alt Fuel Jeep (electric)
'98 windstar 3.8L
'99 Crown Vic 4.6L
'90 Full Size Bronco 5.0L
I'm not a professional mechanic, take my suggestions with a grain of salt, or a cup of coffee 
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