Rus22

Among other issues with my 2004 ford F150 SC 4x4 I now have a transmission issue. I came up behind a slow vehicle that was turning and I pulled out to pass and when I did the tranny downshifted to pass and when it did it popped right back out of passing gear and I thought I heard a pop. I tried to get on it afterwards on straight flat ground and it won't downshift. Rpms go up normally and it just shifts up thru the gears but never downshifts. Could it be something external(sensor,etc.) or internal to the tranny????