[68Timber]

It sounds like the switch is controlling lockup of the torque converter. Easy enough to verify, just get it up to high way speeds with the switch off and at a steady speed, then turn it on and see if you get an rpm drop.

When you say it sounds better at 60 than 80, how do you mean? Wind noise, engine rpm or strain, tire noise, ?

Beautiful truck btw, and welcome to the forum.