No need to dig. I have the full scoop right here!
Jeep NP208 used '80 to '88 in FSJs as an option. All FSJ 999/727 automatics and 6 cyl 4 & 5 speed trans came with 23 spline (same count and depth as the Eagle input) If it's (factory) V8 with T18 4 speed, then it uses a Ford NP208 (31 spline input) because to save costs Jeep went with the Ford trans and case together with a Jeep bell.
Dodge NP208s are 23 spline (except behind some of the the Cummins diesels and V10 which were 29 spline) BUT they are mirror imaged to the Jeep/Eagle tcase.
Chev NP208s are also backwards and slip yoke with the weakest internal components in the 27 spline cases. 32 spline have a stronger, larger input bearing setup (like the Ford one)
Quick recap for those suffering TMI overload:
'80 to '88 FSJs came with the correct NP208 as an option except V8 4 speed.
Quick way to spot '80+ FSJ is look at the front driveshaft. Driver's side is '80+ and pass side is pre '80.
Also note that the front driveshaft of the Eagle will bolt in, but you will need a longer rear one (5" from memory, but it's a bit shaky) and the yokes from your old case may need swapping to fit the Eagle ujoints.
Now who wants a granny low T18a/NP208 combo with a running 360 for $500? Even comes with FSJ pedals and linkages. Trouble is I'm near Seattle.