98 Jeep Grand Cherokee cranks, but won't start

My jeep was running fine one day, later I
leave for work and it won't start. The
battery was pretty dead so I had it checked
and it was fine so I had it recharged. It still
didn't start. Tested the alternator twice and
it's good. It is getting fuel and spark.
Replaced the crankshaft sensor,
distributor, rotor, ignition coil, spark plugs
and cables, tried starting it with a bit of
starting fluid and it would only backfire. I'm
frustrated and out of ideas now. Any help
or other things to try would be greatly
appreciated!