2008 Ford F150 XL 4R75W Transmission Failure with P0720 Code - No Forward Gears, CEL Off

I own a 2008 Ford F150 XL 2WD with a 5.4L engine and 4R75W transmission. This is my father's truck. A few weeks ago, when trying to leave the house, I noticed it required an unusually large amount of throttle to move—eventually it started moving but only after significant effort. During the drive home, the Check Engine Light (CEL) activated with messages indicating 'Reduced Engine Power' and 'Transmission Failure'. I pulled diagnostic codes and found a P0720 fault code: Output Shaft Speed Sensor Circuit. After that, the truck lost all forward gears—no Drive, 2nd, or 1st gear—but reverse function remained fully operational. I replaced the Output Shaft Sensor (OSS) and its connector, inspected the wiring harness, and confirmed no visible damage. The CEL turned off after these repairs, and initially, the truck drove normally through all gears. However, it occasionally felt like it was slipping out of gear or required very high RPMs to engage any forward gear. Now, the issue has returned: the truck again lacks Drive, 2nd, and 1st gears—reverse works perfectly. The most puzzling part is that there are no warning messages on the dashboard and no active check engine light or stored codes. I've already replaced the Transmission Range Sensor (TRS), but the problem persists. My next step is to replace the Turbine/Input Shaft Speed Sensor, which I plan to do in about two weeks. At this point, I'm unsure what else might be causing this issue. Any additional insight or troubleshooting tips would be greatly appreciated. If I missed any details, please let me know. Thank you for your help.