My speedometer doesn’t work and hasn’t worked since prior to trans trouble. Is there a step by step procedure that I can use to diagnose the speedometer problem.

 

Without a FULL vehicle scan (ALL modules) there is going to be a BUNCH of guessing and wasted time.
BOTH the speedo and gearbox threads you have going would benefit from all the modules read for current content.

The gearbox wants info about speed on the CAN bus so all faults might be related to a common problem?