5th Gear Lockout

[CLB]

Hello,

I'm a bit of a Newbie so not sure this post is in the right section. I have a 2005 F250 4x4 6.0L Auto and love the truck so far. My only complaint is when towing in hilly terrain it constantly hops between 5th and 4th gear in Tow/Haul mode (trailer is about 9000lbs with a large frontal area). In this terrain I would prefer to just have it stay in 4th.

Is there any way to lockout 5th gear so the transmission stays in 4th (1:1) or even 3rd overdrive (1.10:1)?? The gear selector only gives you the option of 3rd or OD.

I would love to somehow have this lockout hooked to one of the upfitter switces.

Any help is appreciated!

Thanks.