You should mark multiple-piece drive shafts before disassembling in order to maintain:

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Maintaining driveshaft phasing is essential when you disassemble a two-piece shaft. The two halves must be reassembled in the same relative orientation so that the universal joints’ planes align correctly. Marking before separation lets you put the halves back together with the original phase, preserving the intended angular relationship between the joints. If the halves are rejoined out of phase, you’ll get vibrations and rough operation at certain speeds because the joint planes no longer rotate in sync.

Length is a fixed dimension set by the shaft design, so marking won’t change the overall length. Balance is handled by proper balancing of the assembled shaft; misphasing mainly causes vibration due to improper joint orientation rather than a balance issue. Alignment is related, but the specific issue addressed by the marks is preserving the correct phasing between halves.