1996 gmc jimmy will turn over but wont crank un less you put eather in it and when it starts up it runs normal but wont start with out eahter

Hmmm so the old boy wont start without some starting fluid eh?
I Love GM vehicles but I am not there. Here is a checklist of what I would ask myself were I there and able to test.
1. are the batteries and starter in good condition so it cranks at the speed it should without killing the voltage necessary to fire the injectors?
2. what is the fuel pressure to the injectors?
3. when was the last tuneup?

Id lean towards the fuel pressure because by this year it started being more critical and the pumps were in the tank and they would start going bad and causing all sorts of hard to troubleshoot problems. These days I pretty much start troubleshooting with checking fuel pressure.
Fuel pressure with the key on engine off should be 60-66. anything less then that could be a problem.