Battery isn't charging as it should and now when trying to crank it is barley starting

Alternator would be the first choice. Auto Zone, Advanced and others will check it at no charge.

Regulated battery voltage should be 14 to 15 volts, with the engine idling. If it's low, the voltage regulator may be bad. On many Ford diesels, they are external and are very easy to replace. They are typically held on by only three Torx 20 screws. This could save you a lot of money.

If the voltage regulator is internal, simply remove the back cover and loosen the three Torx 20 screws securing the regulator. I would also clean the battery terminals and cable connections; fully charge the battery, which most auto parts stores will do for free, too. Failure to charge the battery can burn out a new or rebuilt alternator. Alternators are battery maintainers, not chargers