Your 1987 will have the BA-10/5, I know of no way offhand to convert that to take an external save (I went through four of those gearboxen in my 88! I replaced the clutch just because I was there anyhow...)
The first step in converting to an external slave is sourcing a replacement transmission. The AX-15 and NV3550 found behind the 6-242 are direct swaps.
NB: The transfer case is clocked differently between the XJ/MJ/ZJ and YJ/TJ versions - you can freely put the former in the latter (you just lose some ground clearance,) but going the other way requires either redrilling the tcase adapter housing, getting a clocking ring, sourcing the correct tcase adapter housing, or doing sheetmetal work on the transmission tunnel.
The AX-15 can also be found in V6-equipped Dakotas, but the bellhousing will be wrong - you'll have to source an AMC-pattern bell. The bellhousings will swap freely.
Note, however, that the AX-15 maintained the use of the "internal" slave cylinder through at least 1993, so you'll have to check before you grab. The "internal" slave (AX-15i) may be converted to an "external" slave (AX-15e) by replacing the bellhousing with the correct version, including the clutch linkage parts; and swapping the front bearing retainer for the correct version (the front bearing retainer also carries a sleeve on which the TO bearing rids - if you don't install the correct FBR, your clutch will not work.)
The NV3550 and AX-15e also use the same bellhousing.
Potential Donors -
-1994-1999 XJ 6-242
-1994-1995 YJ 6-242
-1997-1998 TJ 6-242
-1993-1994.5 ZJ 6-242
AX-15i (requires conversion to AX-15e):
-1989.5-1993 XJ 6-242
-1989.5-1992 MJ 6-242
-1989.5-1993 YJ 6-242
-2000-2001 XJ 6-242
-1999-2005 TJ 6-242
The AX-15 is derived from a Toyota design, but I don't recall which. Figure that out, and then check for Toyota donors as well.
Also, as I mentioned, check for the AX-15 in V6-equipped Dakotas of similar vintage to vehicles listed above.
The AX-15i and AX-15e can be readily distinguished visually by checking the driver's side of the bellhousing - if the lines go in through a rubber grommet, it's the AX-15i. If they go directly into the slave cylinder, it's the AX-15e. I believe the AX-15e hydrauilcs terminate at the bellhousing using a rubber line - vice the two steel lines on the AX-15i.
Note that while the 2WD and 4WD versions can be converted one to the other; a full teardown is required so that the mainshaft can be replaced (the 2WD version is about 6" longer) and the tcase adapter housing needs to be sourced to replace the tailcone.