"So, I DO have to change to a high gear to go freeway speeds..."
Is your 54 a 1/2 ton with an open driveline? If so, freeway speeds are achievable by swapping out transmissions for an OD transmission. There are plenty that will nearly bolt in to give you that freeway speed. An MY6 4 speed (A833) would be high on my list.
If you have a closed driveline, you would have to swap both the transmission and the rear axle to achieve this.
