Did you check the ujoints out of the shaft.....with mine they seemed fine but
once out of the van they were shot to hell...and once out of the shaft they
looked even worse...other then that, yes it could be out of balance but that'd
be odd for the original shaft to go out of balance while on the van
....perhaps your vc or tranny.  Had a car that vibrated once and then the
tranny fell... I hope this is not your problem.

Tim G
87 gl syncro