We took our old ('96 4WD CTD) truck to local suspension shop for similar issue couple years ago, messed with it couplke days & decided front caliper bad. recommended replacing both, price wasnt that bad so we had it done.
Driving truck home hit the brakes & truck pulled HARD to the right, almost wrecked... ... Back to the shop. They had it almost 5weeks, rebuilt rears, swapped fronts, tried different pads etc... . Remember, this is a pretty well respected spring shop/ suspension outfit----I do not doubt they were doing everything they could, they lost their @ss on this job as I only paid them for parts that stayed on the truck and the original labor quote!
In the end, the official version was they had to install a brand new set calipers from dealer (they tried 2 sets of rebuilts, swapped around, rebuilt them themselves, etc), OEM pads, from dealer. Truck drove sweet after that. Makes no sense to me either---I think one of the rubber brake lines was bad myself.
good luck!