code po773??wants to slip to neutral from reverse ans low speed... automatic

Question

2000 tundra v-6 auto.. had to replace trans.. with used model.. full flush befor install and used used torque convert.. trans holds in all gears fine. Howver I can only reverse at 1.5 mph then it slips to neutral.. unplug the harness and it is fine you can go 100 mph in reverse,but you must manual shift on the column forward