My 1990 f150 is a 300 I6 with a t18 4 speed manual transmission swapped into it. Tac says 151k miles and no idea when the t18 was swapped into it. I original it was the 5 speed. To describe the issue, the transmission is very hard to shift into gear and most of the time it will vibrate and basically fight back. Getting it up gears isn't too bad bad of a problem. But going down gears, it's nigh impossible no matter how much strength I use.
I have no idea if this is just a transmission fluid thing, if the transmission is bad, if it's something with the clutch
You'll get that kind of problem if the clutch is not fully releasing.
What are some reasons why the clutch wouldn't release full? I do know last owner said 3rd was a bit hard to get into before it sat for 2 years, if that info helps at all.
If it's a hydraulic clutch there could be air in the line or a leak. If it's a cable or mechanical linkage it could be out of adjustment.