I have a 1999 Ford F250 Super Duty with a 5.4L engine and 5-speed manual transmission. It has 261,000 miles but im not sure if its the original transmission or engine. The transmission shifts ok 1-2 and 3-4 but when i shift to 5th gear at low speeds and low RPM it grinds. I can wind the truck up to 55-60mph+ and shift to 5th gear and it goes in real smooth. But if your just lugging around taking it easy it grinds going into 5th. It seems to be getting worse at times. Any suggestions on whats going on? Thank you for you help in advance!

 

5th synchro is worn. Normal for that many miles, but usually hits 2nd and 3rd before 5th, as they usually take more shifts and downshifts.

 

well that has crossed my mind, but i always thought when the synchro was going bad, that it would grind at high RPM and not so bad at low RPM. Mine is the opposite.