error code p0785 solenoid/timing malfunction

My truck's check engine light came on 2-weeks ago and i ignored it.Well now it feels like maybe my tranny is going. It starts off running fine but after it seems to warm up,that's when it seems like it's taking off from a stop in 3rd gear, and on the freeway it won't shift into the last gear.
Does this mean I need a new transmission?What do I do To fix it?

Welcome to AF.
Always give year and make and model and engine and transmission and mileage for help on it.
Has transmission ever been serviced?
Code 785 is the 3-2 shift solenoid circuit or solenoid problem.
But as always code does not say replace solenoid but to check it out.
A good repair manual will have a code 785 repair flow chart in it.
Let us know how it goes.

Welcome to AF.
Always give year and make and model and engine and transmission and mileage for help on it.
Has transmission ever been serviced?
Code 785 is the 3-2 shift solenoid circuit or solenoid problem.
But as always code does not say replace solenoid but to check it out.
A good repair manual will have a code 785 repair flow chart in it.
Let us know how it goes.