when you're not geared properly (31's on a truck that was 28" stock with 4:10's), the only option is to rev up in a lower gear... on some grades I lose speed in 4th but in 3rd I gain speed.
Even if the torque / hp peak has been passed, there's still usable power above 4k if you're in the right gear.
I keep telling people to not be afraid to rev it. It may not 'sound' good but the engine will survive (well, at least mine has 248k+ on the clock).

 

I understand what you are saying, I'm running 31's as well. In western PA we have a ton of those hills. I hit them in 5th and top them in 3rd. I just try not to run over 4.
248K that's twice mine...That's why I bought one, they just keep on going...