Starter won't stay engaged long enough to start the van

not enough power to engage the starter. it might how power to turn on everything but not enough amps to start, have it tested at autozone

Check battery connections and battery fuse. Battery may not be large enough for the size of your van

Check for a loose or corroded ground wire at the battery or from the engine to frame.

check to engagement wire at the starter, make sure that power is only present when key is in the start position.if this is the case, then your sarter bendix is hanging up on the flywheel, this is caused by either inproper shiming where the starter is bolted to the block, or having the wrong bendix on the starter, there or several different bendix s general motors uses, also if the return spring in the solonoid is broken or not strong enough the starter could hang up. if the problem is power continuing to feed the solonoid with the key in the run position you could have a bad or out of adjustment ignition switch, since the power feeds the solonoid directly from the ignition switch, other than the neutral switch on the colomn

I know you had the starter tested but i still thing you have one of two things going on here, first acid damage to the Positive battery cable at the battery, if this is present you will notice the outside of the cable seems swelled yo under the red insulator, this causes low voltage, enough to click the starter solenoid but not enough to turn the starter motor. The other problem is burnt contact inside the starter solenoid itself, this problem will cause a clicking noise from the solenoid with no starter engagement, it very common. The starter solenoid is near the battery just follow the Positive cable from the battery to it, carefully jumper across it, does the starter engage? Here is a picture of the starter solenoid.

The starter motor when cranking the engine when energized, causes the crankshaft to turn and the belt pulleys will also turn. If the starter motor just sounds without engaging, there is a problem with the starter solenoid or the starter great teeth or the ring great teeth.
Once the engine cranks but will not run, the engine must have compassion, fuel and spark for it to run.

So what it only does it in park or neutral ! The vehicle starting circuit is only suppose to let it start in park or neutal . Park neutral safety switch won't allow B+ voltage to S terminal on the starter solenoid unless the vehicle is in park or neutral . It's probably the starter relay contacts stuck closed . You really should learn how it works before guessing as to the problem . Plus learn automotive electrical testing ! The ignition switch when turned to the start position send's a voltage signal to the PCM - engine computer .The PCM will energize the starter relay to crank the engine . PCM / ABS 10amp fuse in the I/P fuse box supply's B+ voltage to the park / neutral safety switch . When in park or neutral the B+ voltage goes through the control side of the relay to the PCM which will supply a ground to energize the starter relay .

It could if battery only has enough power for just the lights, what I would try first is turn the light on and try to start the van, if the lights go to dim or off, then the battery is no good, if they stay bright then more then likely it's going to be your starter, another way to test the starter is hit the starter with a hammer or some thing you can reach it while some one holds the key to start, if it starts or not replace it anyway bad starter.

2 problems, first you have bad starter or starter relay for starter staying engaged, keep alt plugged in has nothing to do with starter staying engauged, to test starter relay, jump at relay bat + post with red/bk wire on solonoid, if cranks over without staying engaged, then not relay or starter but rather in your ign switch, ford had bad problems with ign switch some years ago doing that

The problem you describe is consistant with a faulty starter selenoid. You can check the selenoid by attempting to jump power from the incoming hot selenoid connection to the hot that goes from the selenoid to the starter. Do this with the switch in the "on" postion. If the starter then works, replace the selenoid. Good Luck! I hope you find this to be very helpful!