2008 Ford Explorer 4WD Stuck in 1st Gear with P0731 Code - Transmission Shift Issue

Hi everyone! I recently purchased a 2008 Ford Explorer 4WD with a 5R55S transmission. The transmission was rebuilt at 146,000 miles and now has 192,000 miles total. It's been serviced before—last overhaul occurred around 45,000 miles, where the shop replaced the solenoid body, pump valve, torque converter, installed two sleeves, and replaced three bands along with the overdrive plate (planet gear). The vehicle now exhibits a persistent issue: it hangs in first gear until I either release the accelerator or accelerate aggressively to near redline—only then does it shift into second gear. This behavior is inconsistent and causes significant driving frustration. The dashboard warning light shows a wrench icon, and the P0731 diagnostic code appears intermittently. I've tested the transmission with a local mechanic who used a scan tool and reported that 'all gears are gone'—but I can clearly feel each gear shifting from 1st through 5th. Reverse engages smoothly, and all other gears function normally. The truck also now shows a strange behavior: after putting it into drive with overdrive turned off, the dashboard O/D light turns completely dark five minutes later. I'm trying to avoid a full transmission rebuild (which would cost around $2,200), as I only need resolution for the 1st-to-2nd gear shift delay. I don't want to drive slowly or risk backing up traffic due to sluggish acceleration. After clearing the P0731 code, it reappears on its own during subsequent drives. I'm seeking advice from experienced users—any insights into what might be causing this specific issue with the 5R55S transmission? I plan to get a second opinion but would appreciate any technical pointers before investing in repairs. Thanks for your help! Best regards, Jordan