2003 F-150
4.2L
140K
stick shift

For about 1500 miles, my truck has been substantially harder to shift than normal. Today, while on the highway, it began making a high pitched whine - much louder in lower gears. Now, it is nearly impossible to shift...any ideas?

 

Could be syncronizer(s) or most likely the clutch is worn beyond limits.Another possibility is a weak Clutch Master Cylinder.Im not familiar with Ford Standard trannys,but must assume todays trucks have Hydraulic Clutch activation.
This being the case,check the fluid level in the Clutch Resirvoir,and if its normal,you might want to "Bleed" the line that goes from the master to the slave cylinder.This is accomplished just as it would be if you were doing brake bleeding.
IF the fluid system is good..no air in the line,and the slave is moving the clutch arm properly,you have a worn throw-out bearing,clutch assy, or a combination of both. My syncronizer theory probably is not a factor,but bears keeping in mind..

 

Good advice above.

Hydraulic master cyl at fire wall and slave cylinder on transmission. Check fluid level.
If all looks OK external, then suspect clutch/throw out bearing and then transmission internals.

If you can, put it in 4th gear and cruise along at mid RPM levels, find the RPM range where it is starting to make decent power, then gas it hard. If the engine revs and the truck doesn't speed up, then you have a worn out clutch assembly.

 

Hey folks,
Thanks for the responses - much appreciated. I took it to a tranny shop, and the problem ended up being a broken slave and a worn clutch disk/pressure plate. Shifts better now than it did when it was new!