Figure out the year of your Jeep, manufacture date of the T-case(small circular metal tag on back side), and what transmission and T-case it has. We should be able to figure it out, although there's no way to be certain without measuring.
For example: I know mine is the .750" input shaft without measuring because it is a 93ZJ with I6 and AW4 tranny/249 T-case. I'll look for the info I used to figure it out. All I did was look at all the T-case swap info google could find for me.
EDIT: Here ya go........guess that doesn't really help huh? .
93-95 V8 ZJ's: 46RH transmission, 1.55" 23 spline input shaft
93-95 I6 ZJ's: Used both the .750" and .840" 23 spline input with the 42RE and AX15.
93 I6 ZJ's with the AW4: .750" 23 spline input
All 96-98 ZJ's: .840" 23 spline input
I found some good info for you.
(the following taken from the previous link)
Input Gears and Compatibility
Hold on to your seat...
The important variations to look for in the 231 transfer case are in the input gear. The 231 comes in 21 spline and 23 spline versions with long, medium (rare) and short variations.
The long shafts protrude 2.1" from the front mounting face of the transfer case. The short version protrudes 1.2". The rare medium shaft protrudes 1.7". You should be careful when swapping any of these gears due to different bearing widths and gear teeth. The following chart shows how to determine which input gear you might have by transmission. It is always recommended to verify first hand if possible.
21 Spline 23 Spline
Short Input AW4 (through 1990)
Peugeot BA 10/5 (earlier) Earlier (pre-'95) AX15
Long Input Later (post-'94) AX15
Peugeot BA 10/5(later)
Medium Input 46RE Jeep ZJ's '93 to '95 only
Because of differences in the cut of the gear, as a general rule, one should not interchange 1987-1993 gears with 1994-present gears. These years are not always hard and fast delineations. If one is not absolutely certain, disassembly and inspection is required.
Note that some transmissions change spline count. The Peugeot BA10/5 had both 21 or 23 splines. The AW4 had 21 splines through 1990. In 1991 (the same year the 4.0 HO was introduced) the AW4 splines increased to 23.
ReEDIT:Looks like all 97ZJs have the .840" shaft. I'd look for one that is already pulled and measure the exposed part of the shaft. If you have a 97 and you go with an older one you may have to also swap rear driveshafts. I'm going from a 93 249 case to a 97 242 case and I need to change rear driveshafts. 93 has internal slip yoke, and 97 has external. I hope some of this was helpful.