Hello, I have. a 1991 For F150 4.9 I6 with an automatic transmission 4x4. I have noticed over the past few years that the truck has allot of problems maintaining highway speeds over 55MPH. When it does get above 55 the truck begins to shake. It does appear to shift. To be honest Im not sure what Transmission it has in it. I had the engine tuned up this past summer and cylinder no 5 was not firing. The mechanic replaced the Distributor Cap and all the plugs. They had no way to check the Fuel injectors. What can cause a lack of maintaining highway speeds. I pulled codes and it showed. 172 R sensor fault lean. Im not sure if it is correctly pulling the codes because I have another engine light on for "Rear Antilock".

 

Check your fuel pressure while you're on the freeway. A weak fuel pump will cause a low-top speed and a lean fault code. Here's how to check your fuel pressure. https://www.ford-trucks.com/forums/1...rd-trucks.html
let us know what you find.