most likey cause is the seals going to the front engagement hubs-these are know to go bad-check vacumm to the front hubs-if you have vacumm going to the front hubs then the seals are bad in the front hubs-this requires you to replace the hubs-this is a very common problem-also check the vacumm lines going to the hubs-if no vacumm to the hubs then its the lines or the vacumm cannister or the feed to the cannister-but considering the amount of hubs ive replaced on these trucks id say you have a vacumm leak at the hub seals
hope this helps
gene
ase master certified
ase master certified 40 years in the repair industry