My starter isn't engaging while on vehicle, but it is spinning?
When I take the starter off, It does engage when I jump start it. The truck is a 1986 Ford Bronco. 5.0 Motor. I've tried putting a new ground from the battery to the engine. I also took the starter autozone and they tested it and said it was good. Any advice is appreciated.
That's really weird that when its off the truck the starter gear pops out and spins. But the gear doesn't pop out while on the truck. I'm going to guess and say its a bad starter, and it tested good at autozone because it was banged around a little bit when taken off the truck and in transport causing it to pop out 1 more time when it was tested. If its on the truck right now tap the casing lightly with a hammer to see if it will unstick itself and work. Try again and again.
Problem with a bench test there's not a flywheel in front of the starter to load test against. If the bendix is weak it will still shoot out and turn on a bench test. Weak battery and failed starter solenoid could also be a problem.
Your problem is the starter drive aka Bendix. When the gear engages the flywheel with a bad starter drive it just spins. Here is a picture of the drive for your vehicle. The drive is designed to spin in one direction and hold fast in the other. This lets it disengage from the flywheel and hold in the other direction to turn the engine over. If you turn the gear by hand and it turns both directions the drive is bad. 
Get a new battery.
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