knocking should when accelerating from stop

[nictherom]

Have a 72button k10 everytime I pull away from a stop I hear a click or knock in the rear thought it was my loose tailgate at first but it sounds a lot heavier than just the tailgate bouncing around any ideas on Wat to check first
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[dznucks]

I'd start with the universal joints and the rear differential.

Jack the rear of the truck up and put jacks under the axles. Spin the tires and see if you hear or feel anything knocking. turn the driveshaft (transmission in nuetral) and feel/listen for the same thing. If you havent found it yet, lock the rear tires (parking brake or helper) and then try to rotate driveshaft.

If you dont find anything look elsewhere.