Only way to make the u-joints go out of phase is to disassemble the driveshaft at the splines. Not something that you normally would need to do.... There is no way of suddenly making them "jump out of phase", it is an assembly issue.

If the problem has developed without doing anything to alter the vehicle in any way then it is not a reassembly issue (out of phase u-joint). If the vehicle does not exhibit the same symptoms throughout the speed range, then it is unlikely to be something that is actually broken. If the vehicle only develops the symptoms at a particular speed (range) then it would most likely be a balance issue (tyres/driveshaft balance), or a mechanical resonance/balance issue (idler arm bushing, bad u-joint causing driveshaft vibrations, loose wheelbearing with a bit of "caster shimmy" [ie, "shopping trolly syndrome"]).

That's the logic behind my thinking.....