i have a 89 cummins and when i drive it makes a bad noise like im driveing on rummble strips. if i lock my hubs in it wont do it at all. any ideas?

 

Id pull the hubs and look at the bearings. I think what may be happening is when you engage the hubs, the hub is now supporting the axle shaft instead of a worn out spindle bearing. Take a look at the axle u joints as well while you have it apart.