Is it an E350 or E450 chassis ?
My E-series has a button on the end of the gear selector to cancel Overdrive, which I always do when approaching a hill.
Do not be afraid to downshift to second gear. Just watch the temperature gauge and pull over if it is in the HOT range. If you are really paranoid, have a shop install a transmission fluid temperature gauge.
Worst case, find a shop that specializes in rear axles and ring and pinion gears. If you do NOT have a 4.10 rear axle ratio, installing it may be worth the cost is you travel in the hills a lot.
