I'm going to just try to get to the point bc posting on my iPhone drives me nuts. I posted this on f150forums and got no reply so I'm trying here. I bought a 99 ext cab f150 4x4 with a 4.6 and 4r70w for cheap. The trans was slipping bad in od which apparently happens alot with these trucks. I was doing yardwork, had the truck in 4low and 1st, went to drive off and BANG! The truck started rolling backwards in gear. Tried all gears and nothing. Shut it off, put in park, started up and now I have all gears and 4wd ranges. Revving the truck it has a shutter now which feels like a miss until like 2500+rpm. Took it down my road and shifted to 2nd fine but once we hit a slight incline it started to shudder and really couldn't make it. I drained the fluids from the tc and dropped the pan, no metal chunks at all and fluid actually looked good. The magnet was 4times it's size with metallic sludge tho.

 

Probably a bad torque converter and/or transmission. Either way it's got to come apart.