I do mtn driving...plenty of 6% grades.
leave it in overdrive and use the brakes...well the brakes will burn up and fail when you need them.
Overdrive is fine for going uphill, going downhill ( like when you see the sign which advises trucks to use lower gear...shift into 3 or even 2...think about controlling your downhill speed before your nose is pointed down the hill...what is safe.
I have a 2006 PW Lexor on a Chev Chassis. 6.0 liter ( 305hp) and 4 speed ( often referred to as a 3 speed with Overdrive)- this IS a well matched drivetrain/gearing to the weight and size of the van...other vans may not have as good mechanical advantage
Generally 3rd will hold the van to about 55 on a 6% grade. 2nd will hold to about 45 mph.
using this approach if you come around a curve and there is a school bus laying across the road...your brakes will be cool and fresh giving optimal braking power.
And the pads will last longer
and the disks won't be warped
if you find the tranny is "hunting" going up hill- changing up a gear, then downshifting for a minute, then upshifting- downshift manually and just leave it there until the grade isn;t as steep
you can alter the transmission shift points with a "tuner" and to an extent using the tow/haul mode button- which will generally allow the motor to rev higher before changing up into the next gear (this will vary greatly by model motor/ tranny...the tow/haul on my gmc truck does nothing...)
On GM's the tow/haul button is sometimes on the dash with a pic of a trailer...sometimes a buttom on the end of the gear selector
many of these vans do not have a tachometer- if you are concerned about revs, you can get a unit which plugs into the OBD data port and will display rpms and other data..
mike