* Transmission size and mounting points: The Ranger's transmission is physically smaller and likely has different mounting points than the F-150's transmission. You'd need to check for compatibility precisely. It's highly improbable they'd line up.
* Torque capacity: The F-150 likely had a much larger and more powerful engine than the Ranger, meaning the Ranger's transmission might not be able to handle the torque. This could lead to catastrophic failure.
* Driveshaft: The driveshaft lengths and yokes are almost certainly different. You'd need a custom driveshaft or at minimum a different yoke.
* Computer and wiring: The transmission control systems are probably different and incompatible. Significant wiring harness modifications or an entirely new control system would be necessary.
* Bellhousing: The bellhousing, which connects the transmission to the engine, is almost certainly a different pattern.
In short, while theoretically *possible* with extensive modifications, it's not practical or cost-effective. It would be far simpler, cheaper, and more reliable to find a used automatic transmission from a compatible 1990 F-150 or a similar year model. The amount of fabrication and custom work required to adapt a Ranger transmission would likely far exceed the cost of a correct replacement.
