Way does transmission case brake on Isuzu npr?

I have had this happen 4 times to me on one truck! What you have to do is send the drive-line out WITH THE PARK BRAKE DRUM ATTACHED as well and get it balanced. I sent just the d-line out for balancing and it happened again!!. So we sent the parking brake drum along with the d-line and haven't seen truck sense. Trust me been their done that. Let me guess the engine is a gasoline powered 5.7L Chevy 350?? Ya it is!

Hope this helps. Good luck.