2003f150xlt5.4l-4×4 Transmission won't engage. Driving down the road in normal conditions and the transmission started slipping and would not engage. Checked fluid level and had to add fluid. Truck took off no problem. Got a mile down the road and transmission slipping and would not engage. Checked for leeks. None found. Please help!!! Truck is my only vehicle and I LOVE MY TRUCK!!!!!

 

So the fluid is between ADD and FULL on the stick with the engine running in Park? Do you see any leaks - a puddle of fluid under the truck?