You cannot hurt an automatic by downshifting - you can even yank it down into 1st gear at 70 and it's not going to downshift until the computer deems it safe. It will downshift as far as it can, then when you slow down it will keep downshifting at the appropriate time.

A moving truck may have what's called a "tow/haul" mode - it uses a different program which will downshift for you if you hit the brakes on a downgrade.