Matt - A thorough going over of your suspension (springs, bushings, shocks etc.) is certainly called for to see where your truck is condition wise. Also worth checking and rebuilding those cab mounts if they warrant the work. Another source to check is your existing tires for type (bias or radial), age and any unusual wear patterns. Don't forget to check their current state of balance. Bias ply tires are going to give a much stiffer/bouncier ride than radials. Plus if they have sat in one place for long periods of time, they can develop flat spots that lead to very bumpy rides.
Also of note is our trucks weren't really designed for higher speeds. The original suspension and steering is going to give a less comfortable ride when pushed up above the normal original operating range, especially if their is any wear or condition issues.
