truck questions

[Susan Evans Garlinghouse <suendavid@worldnet.att.net>]

This may be a dumb question, but does anyone know if it's possible to
add an additional "granny gear" to an existing transmission?

We've started pulling three horses in our (admittedly very heavy) rig
and although the truck has plenty of power for pulling on the flat and
pulling up hills on the highway, the trails we've been going to include
pulling up a long, steep grade from usually a dead stop at the bottom. 
I just can't keep my rpms up high enough and the truck is having a
really hard time getting up the hill, as in close to lugging out near
the top.  I can of course just not go to this trail, but there are other
steep hills and you never know when you'll have to climb one.

When I used to have my 4x4, I could crawl up ANY hill from a dead stop
by using the low gears, but this truck (though much more powerful) is an
automatic HD 4-speed OD (no, I don't climb hills in OD) and 1st gear
just doesn't seem to be low enough.

The rear end differential is a 3.73, the engine is a Chevy supercharged
454 dually.

Any suggestions on solving this problem, short of tying the third horse
to the bumper and making him get up the hill on his own, buying a new
truck and/or trailer?

Thanks,

Susan G