You can take the nuts off of the front cab bolts and use a pry bar to lift the cab to get a better look at your front cab supports to see if one of them is collapsed or completely missing. The rear shackles will articulate enough to allow this to happen.
If the rubber is sound in your rear shackle mounts they will be fine even if the rubber is hard as a carp.
Your trouble is up front more than likely.
