If you have a known good vm in there then that obiviously is not the issue, pins in connectors/wiring is still suspect.

Through my experiences with this truck sometimes you replace what tests bad on the lift heatsink and may miss another component that you don't have the ability to test due to other parts being bad. Key on and your new parts are toast and end up right back where you started. The lift heatsink used to be sold as an assembly and it was much easier then. Now you can buy all the bits and pieces on it...
There is no 1 part that causes 6r it's sort of a catch all for the transistor controlled lift system.
All that being said I assume you have the service manual, I suggest you follow the troubleshooting outline for 6r and reply back with your results, also there are other tests for the lift system not listed under 6r troubleshooting so check out any other tests that may be relevant and conduct them.

This is no cake walk, I have watched dealer techs have this code get out of hand fast with the vm being in excess of $5000 and heat sink above $2000. Proper testing is required.