Missing coolant is never good. Hopefully it is a leak in the plastic end tanks but with none showing on the groung chances are it is a cracked head. Try to find a buddy with a pressure tester to verify a crack in the head or pull the plugs and see if they are discolored, easy way to see if you are burning coolant. I had a ranger that drove perfectly fine and happy with a crack for many, many miles(2 years) Of course it was a beater truck and I didn't really care if it ate a set of rings. It also did not put out the sweet smell you assume it would with burning coolant. The heads were eventually pulled and it had a huge crack in it. Has the truck ever been overheated ?