When the truck is not running I can shift gears, but when I start the engine I cannot shift gears. Is there some type of inter-lock?

 

No, there is no interlock.

When you are able to shift gears with the engine not running, are you doing anything with the clutch?

Is this problem something that has been building up gradually or did it just all-of-a-sudden appear?

Assuming you start the engine with the transmission in neutral, then do you depress the clutch pedal and then attempt to engage some gear? Does the shift lever move to the position for some gear, i.e. 1st or reverse or 4th, or does the lever act like it is blocked?

If it acts likely is blocked, check the level of transmission fluid. Note that the Ranger manual transmission uses Ford's Mercon ATF. If the fluid level is low, fill the tranny. Then start looking for a leak.

The first place to look for a leak is at the 3 rubber plugs at the top rear of the tranny to see if the shift rail plugs need to be replaced. They dry out with age and leak. This is a common problem with these transmissions.

I just went through getting the plugs in my truck replaced. My Ford dealer had no idea what I was talking about when I asked them to check them. The service writer, service manager, mechanic and the parts guy all said they never heard of this problem - they hadn't a clue.

Let us know what you find out.

 

Sounds like clutch is not releasing. Check fluid in clutch master cylinder if low fill and pump several times see if this helps if it does you need to find your leak probable slave cylinder leak

 

transmission synchronizer are preventing the gears from meshing unless the gears are near or at zero rpm's. Repair clutch.

 

https://www.ford-trucks.com/forums/9...l-bracket.html

 

If you have left the Ranger parked for a while, and it has rained or gotten wet, try starting the engine with the transmission in gear and the clutch pedal depressed. BE WARY as it starts, it will jump the truck forward or backward depending on the gear you select.
I have had the clutch disc stick to the flywheel a few times, and the above procedure seems to break it loose.
tom