1990 Ford Bronco II won't go over 35 full throttle, but we jacked the rear up and it ran fine.

Sounds like it is not getting enough fuel. Jacking up the wheels removes the load from the engine. Have you checked to see if the exhaust system or catalytic converter was plugged?

Or maybe it’s low on fuel, the pickup in the tank is toward the front sooo, or maybe when u jack the rear and it runs better the fuel is moving toward the fuel pick up screen (clogged?) and more fuel pushed tank dirt away from screen.