Hello, I'm Chris.I will do my best to answer your question fast and accurate using the info you have provided. Thanks for visiting Just Answer.
When you reach the top of 3rd gear, does it try to shift to 4th and slip or just rev up, or does it not even try to shift?
Any warning indicators like the OD cancel light solid or flashing?
You have a 4R100, which has only 4 forward gears and one reverse gear.
Your step one is to use a scan tool and retrieve the stored codes since the computer is trying to tell you whats wrong. It may show a gear ratio error code or a solenoid code. Then your next step is diagnosing those codes. Odds are its going to be the overdrive clutch at fault which means it will need to be removed and overhauled to repair the clutch discs or the pistons seals. The 2nd most common part is a faulty overdrive one way clutch, 3rd is the 3-4 shift valve binding and 4th is the solenoid body.
You can download the info from waht they say it can be HERE