I've never seen or heard of a start enable relay problem in these vehicles so I'm gonna go with the odds and say that'* not the problem either. I hope it'* not cause if it were, the dash would have to be removed to replace.
Have you tried starting it in neutral? There is an adjustment on the gear selection linkage which, if not properly adjusted, could inhibit starting.
Grab a test light, a helper and a print out of the above diagram. First, test the yellow wire under the dash to check the ignition switch. Next, test the yellow wire at the trans connector. This will test the start enable relay. Then test the purple wire at the trans connector. This will test the trans switch.
Since this is an intermittent problem, you will need to do the testing when the problem is present.