I live in BC Canada and the truck is a 1990 4.0 4x4 and occasionally the throttle will stick, I had a look under the hood and removed the little plastic "engine cover" (not sure what to call it as it only covers the throttle body) as that seemed to be what the throttle linkage was catching on. I thought that would solve the problem but it happened again so I did a bit of googling and found that there was a recall in 1990 on the air inlet hose which supposedly is the problem. After I found that I went to my local Ford dealer to see if it was still possible to get my hands on the redesigned air inlet hose that was fords fix for the recall but the truck is too old to be in their new system so one of their older employees searched an earlier database but couldn't find any info on the truck. His computer just told him the year of the truck when he entered the VIN but couldn't tell him anything else. He thought that maybe his system couldn't properly decode the VIN because the truck came up from the states but that doesn't make sense because the speedo is in km/h not mp/h and as far as I know the truck has always been in Canada. Does anyone know more about this recall?

1990 Ford Ranger Recalls - Cars.com

In the words of the Ford tech I talked to "I know why I can't find your truck in my system. Because its an anomaly that shouldn't still be running' lol.

 

Is it sticking at idle when you step on the pedal or hanging open when letting up? If it is sticking at idle the throttle plate may be getting stuck on the throttle body. I know the later throttle bodies have a screw that can be adjusted to keep this from happening. I looks like an idle adjustment screw but it is not to be used for that.

 

It happens when I let off the throttle usually after being at or close to full throttle like when I'm overtaking or speeding up to match traffic speed.

 

I assume you have tried to lube the throttle cable, spray some lube down the cable from engine end.
A loose/disconnected ground strap(drivers side head to firewall) will cause throttle cable to become a cab/engine Ground this dries out the lube in the cable.

Check that the spring on the throttle linkage isn't broken, it is circular, like a clock spring, you can add an external spring for now, throttle must have a spring to pull it closed against air flow/pressure.
If you have cruise control disconnect that cable from throttle linkage for now, see if problem goes away.

See if you can reproduce the stuck throttle with engine off, i.e. open throttle manually and slide it back and forth to see if a bushing is worn out causing it to stick.
Also have some one push gas pedal to open throttle, if it only sticks when using gas pedal then cable is the issue, broken strand in the cable is sticking, same can happen on speedo cable causing needle to jump around.

Opposite the throttle cable side on the throttle plate shaft is the TPS(throttle position sensor) it could be worn and causing throttle to stick.
If you can reproduce stuck throttle loosen TPS and if throttle releases replace TPS