Situation: sitting at a traffic light, light turns green. When I accelerate vehicle won't move. After a few seconds it moves, speedometer not workin, wrench symbol is illuminated and won't shift beyond 3rd gear.

2013, 5.0L, RCSB, 4x4 STX

 

Had a similar issue but was stuck in 5th and wouldn't downshift. Take a look here. If it's similar, there is a TSB to replace the leadframe in the transmission

https://www.ford-trucks.com/forums/1360407-lost-speedo-odometer-stuck-in-5th-no-power-and-airbag-light.html

 

The first, and easiest, step is to check for codes. If you don't have a scanner, stop by one of the many auto parts stores that will do it for free.