can you replace an automatic transfer case with a manual one
 

i have a 91 ranger and the electrical 4x4 keeps messing up can i put a manual in its place

yes you can

manual transfer cases are far better than any chain driven ,, the difference is night and day

You do know the cases in the rangers are all chain drives right?

I think your confusing gear driven and manual cases. Check the table in the middle of the page.

Ford Ranger Transfer Cases

The manual case in the rangers just eliminates the electronic engagement, otherwise they are identical.

it depends which type of manual transfer case he is referring to.

he might just be looking for a manual gear driven transfer case

which are available with an adapter