Now that it has got REAL cold out my 1996 jimmy starter wont go.

It could be a bunch of things I would check the fuel pump relay first its located inside the glove box behind a panal that says relays its the one on the far left . The way the fuel system on this vehicle works is the fuel pump will still work if the relay is bad because the system it backed up through the oil pressure sending unit. That is what actualy sends the voltage to the fuel pump however the sending unit has a switch inside of it that needs the oil pressure to be high enough to trip the switch to send this voltage to the fuel pump. On a cold day the oil pressure can't build high enough to trip this switch so you get no fuel this it why they tell you to put 5 30 wight oil in the engine. This isn't the olny possible answer to your problem however its what i would look at first and if your running 10 30 i would also replace the oil pressure sending unit. Im assumeing that the vehicle is cranking but not starting from what you have said ?but let me know i know how this vehicle works