WARNING! DANGEROUS CONDITION!
We now know what was causing the bucking and vibration when cresting a hill and sometimes when climbing a hill - and it was not a Cummins engine program problem like WRV told us or an Allison computer chip problem like our dealer's mechanic told us and this forum suggests.
Our U-joint failed and the driveshaft came apart and took out the emergency brake along with it as it flew away. POW we were suddenly free-wheeling down the highway - fortunately an Interstate and not the curvy mountain roads we had been on for the previous three weeks or we might have ended up upside down at the bottom of a ravine! After $3,000 repair for new drive shaft, U-joint, emergency brake air valve and transmission bracket replacement while we camped in the junk yard behind a truck repair place, we no longer have any vibration or bucking.
I STRONGLY recommend that anyone with this problem have their U-joint and drive shaft carefully inspected rather than continue driving while waiting for a "computer chip fix" from Allison or Cummins. The mechanic suspected an allignment error or design problem may have been at fault.