I just did the whole clutch replacement on my 90 4x4 dlx pu. Main seal, gasket for the housing, flywheel, clutch, pressure plate, fork and boot, pivot stud, pilot and throw out bearings, hours of grease and oil cleaning, new master and slave cylinders...
everything went back together great, was super happy until while reinstalling the selector levers I discovered the spring in the t-case for the lever wasn't there! I felt around to see if it had just shifted, but no luck. Can that spring fall down inside the works by only removing the shifter or was it just gone to begin with? Please don't tell me I have to pull my tranny!

 

Can you go to epc data and get a screenshot of the spring you’re referring to?

 

its the shifter position return spring at the very top, it pushes the bottom of the lever in H4

 

My 87 doesn't have that spring.
My only thoughts would be to try and look down the opening where the lever installs and see if you can see anything.(assuming you've already done that, though)
Maybe drain the fluid and see if you can see anything through the drain hole. Maybe pour some solvent in through the top to force the spring to the bottom.
Have someone slowly turn the output shaft and if it hangs up or if you can hear something rattling around.
get a flexible stick magnet and run it down. With all the surrounding metal not sure how that'd work.

I'd be curious to know how easy it would be to accidentally pop that spring off when removing. Maybe someone here who's has the same unit can advise.