2004 4.0 L 4x4 5spd tranny that is only able to operate in 4th gear. Spinning sound in all other gears, including reverse, but not engaging into gear. Any ideas? Could it be just from lack of tranny fluid? Hard tranny to find a used replacement

 

Welcome to the forum

In 2004 automatic and manual are BOTH 5-speeds............................................ ...

I will assume Manual trans from description of the problem
You have an M5OD-R1HD(M5R1) in a 2004 4.0l, they came in 2WD and 4x4 versions, not interchangeable
M5OD-R1 is a MODEL number, NOT what it fits, ALL Rangers from 1993 and up used M5OD-R1 with all engine sizes, but these are engine specific and 2WD or 4x4 specific

Your description describes exactly what would happen if Input shaft Gear was stripped
1, 2, 3, 5, and R use the input shaft gear
4th does not, its a direct link to output shaft, 1:1 ratio, so would still work

So no not a low fluid issue, you can drain the fluid and you will find metal bits which would confirm this "best guess"
Drain plug on drivers side

Its a rare occurrence with the M5R1 transmissions, but does happen
Requires a full rebuild to repair

Try https://www.car-part.com/

Good place to look for LOCAL used parts, use your Zipcode
You need a 2001 or later 4.0l 4x4 M5R1 trans, rest of the numbers don't matter

Heads up
These M5R1 MANUAL transmissions use Ford ATF, yes automatic transmission fluid in a manual transmission
People often put in Gear Oil, which is way to thick and will cause issues
Mercon V ATF is spec or synthetic equivalent