ludoggiez....If you have the original coolant, it requires that you test it twice a year with test strips to make sure your SCA's/DCA's are correct. If your going to change the coolant, it's a good opportunity to go with the coolant that is now being used. It's called Organic Acid Technology (OAT) coolant and never needs to be tested and will actually run a few degrees cooler.
If you decide to change to this coolant, you need to turn on your dash heater and drain the coolant. Fill with fresh water, let it get warm and drain again. To really do it right, partially fill it again (6-7 gallons) with distilled water, run the engine for a few minutes and drain. It is not recommended to fill a diesel engine with tap water.
There are several companies making OAT coolant. The EAS Compleat noted above, Final Charge and some others. I don't know your capacity, but buy a couple of gallons of concentrated coolant and the rest 50/50 mix. The reason for this is that you'll never drain all of the water out. Put two gallons of concentrate in first and then fill until full with 50/50 mix. This will insure that you have pretty close to a 50/50 mix when done.