Im having trouble with sheet rock joints cracking and tape peeling in the corners of a vaulted ceiling. They occur where it runs up the rafter to where it levels out at a collar tie. Any help?

One of two things or maybe both. You have a poor installation of mud that never got a proper bite onto the drywall. You could also have a small amount of movement due to expansion/contraction of construction materials or loose nails.

A few questions is there heat vents where the cracking is are the cracks in the corners you should be able to see what type of tape was used ether paper or mesh can you push up on the sheets if so do they move and if they do move you should see the nails or screws pop if you could answer these questions I can help you with this problem