Wetfeets

I have a 00 grand cherokee Laredo 4.0 I bought a couple weeks back from a guy I work with. It had a blown engine so we pulled it as soon as I got it home. We replaced it with a 4.0 out of a 99 4wd I have so I know the engine is good. While swapping these engines I changed the trans fluid and filter, the engine oil and filter, the spark plugs, the valve cover gasket, and put the flywheel off the 2wd engine on the one I put in oh and I installed a new crank position sensor, and the oil sending unit. I got everything done and it fired rite up and ran for about 30 seconds then cut off. I've tried everything I know to do but all it will do now is turn over and spit back through the intake. I checked all the vaccum lines and they are ok as far as I can tell and there's gas at the fuel rail. I'm broke and all out of ideas. Me and my family need this thing going bad we're counting on someone smarter than me to help out. Any help will be greatly appreciated! Thanks!