the truck is a 87... does it have the old style heads or the new thicker castings? The old heads crack real easy and they had a redesign.
look here for info on how to check:
http://www.therangerstation.com/tech_library/2_9_Page.shtml
The quickest way to tell what heads you have is the outer edge where the valve cover seals on above the exhaust. The new heads are more straight than the old style that was really curvey where it goes around the valve springs.
How did the old oil look when you changed it? clean? dirty? or milky like water mixed in?
Thats some serious oil loss. need to find out where it is going.
is it a external leak going on the ground?
or a internal leak into the cylinders and burining it?
might be a head gasket leak or cracked head and getting into the coolant system somehow?
Check the radiator antifreeze and see if it looks like oil is mixed in.
Check to see if there is fresh oil leaking on or under the motor slinging onto the chassis.
If not It has to be burning it and smoking out the tail pipe.
that would foul out the plugs and cause a nasty miss.
That could be caused by bad piston rings, a hole in a piston, or getting sucked into the intake through the PCV valve.
Need to get a compression check on the engine!
very simple to do if you have not done one before and will tell you alot about the condition of the engine.
where about are you located?