I HAVE TO CHANGE OUT A CRANK SENSOR IN MY GRAND CHEROKEE.

An engine needs 3 things to run. Fuel, Air and spark. You've checked the spark so move onto the air and fuel system. I'm assuming you did a full service on the replacement engine after you fitted it? Even a dirty Air filter can cause bigger issue that you would expect.
Have you checked to see if the engine is getting Fuel? If it is just turning over and not trying it might not be getting fuel to the cylinders. When you checked the plugs for a spark were they wet and smell of fuel when you first took them out?
Have you had the vehicle scanned with an OBD2 scanner for trouble codes? These can give clues to what and where the problem might be. If you have to pay for the scan get prices at 4 or 5 different places as prices can be VERY different.

You need a quarter inch ratchet and a socket to fit the two bolts...You will also need about 100lbs of patience (it's a real pain to reach)
Good luick

Likely the sensor involved is the crank position sensor, next in line would be the cam sensor. You could also have a bad fuel pump or pump relay. Crank sensor is on the bellhousing at about 11 o'clock looking from the rear. It is even with the position of the flywheel. cam sensor is inside or is your distributor, depending upon your system. All are expensive so I recommend that you run some tests outlined in haynes, chiltons or other decent repair manuals before changing anything.

Crank sensor is generally located on the bellhousing 11 o'clock position looking from the rear. It is always even with the flywheel as it gets it's trigger from that.
It is not easy to reach it but it is not impossible to do. If yours has spacers, remember that it is better to use too many than not enough (too close to the flywheel and it will self-destruct) it can pick up a signal nearly 1/4" away from wheel though. Since it's not easy to change, make sure you test it before you decide to replace it though!

my 1996 jeep grand cherokee lmtd. won't start did tune up. cranks but no fire. please help. engine code 54 shows up . cam shaft positioning no signal. . tested all circuit. changed cam shaft positioning sensor twice.still cranks did start once ran for 5 min. changed the sensor again. no luck. help please. keith

On the back of the engine, under the firewall, driver side. You can barely see it from above. You have to change it from underneath.

the crankshaft position sensor controls spark and if faulty would give the symptons you have described you need to get this checked first

The crank sensor is located on the passenger side . On 6cyl it attach to bellhousing and on V-8 it attach to engine, just behind the cylinder head

You say you changed the cam sensor. A1391 code is a crank sensor on the top of the trans. bell housing.