Starter going bad?
 

Any ideas on what would make a starter turn the engine over about 2 times then the solenoid let go and let the starter spin while the starter gear is not on the flexplate?

Failing starter. :D

Autozone tested about 5 times and passed everytime.

Yeah, on a test bench....

Check voltage at the little wire to the solenoid. You should see a minimum of 9 volts there when you hit the key.

Sounds to me like where the bendix arm engages into the solenoid, may have failed. It can push the gear out on the test bench because there is no drive restriction. Once the ring gear is in place, the force of the bendix is not enough to hold the pinion gear extended.

Thats what I'm thinking too. I just wonder if it would be better to rebuild or replace this "brand new starter"

If you have a starter/alternator shop nearby, have it rebuild. You didn't say which motor/trans combination, but pretty sure the 3.9/5.2/5.9 42/44/46RE combinations all use a fairly common nippendenso style solenoid and housing.

If it's brand new, should still have warranty on it?

What OP is saying is that they are testing the starter in their POS "test system" at the box parts store, and since it actuates properly, they are not calling it defective to consider it warrantyable

Bring 'em the truck, show 'em the problem. See if that will change their tune. (if possible....)