They BOTH need to be 4x4s
Yes the 2.8l, 2.9l and 4.0l share the same block, so same transmission bolt patterns
Both A4LDs will be the same length, and you use the transfer case from the 1987, so no driveline changes
1987 will be a BW1350, 1992 BW1354, same bolt pattern and shaft to transmission
1987 computer will probably use a 2 wire plug at the A4LD
1992 A4LD needs a 3 wire plug in, 3rd wire is to activate OverDrive(OD)
Seen here:
https://www.explorerforum.com/forums/media/a4ld-specification-chart-from-transgo.39537/full
There is a 12volt key on wire(red) thats shared by the 2 solenoids inside the 1992 A4LD
You will need to spice the 1987 red wire to center pin
And splice the 1987 TCC wire to the TCC pin on the 1992
Then run a wire into the cab to a switch thats grounded to cab metal, so you can manually turn on OD when you want
Off Roaders often added this switch so they could turn on OD manually in any other gear to get "split gear" ratios, in 2WD, 4hi or 4low
Ford did this on the 5R55E as well, the computer would turn on OD in 1st gear to get a smoother shift to 2nd
If you have manual locking hubs and they are locked then ALL transfer cases are "shift on the fly"
"Shift on the fly" only applied to vehicles with "Auto locking" hubs
When both front wheels/hubs are locked then both front and rear drive shafts are spinning at the same speed, so the gears inside the transfer case are spinning at the same speed, but the front drive is just not connected/locked to the rear/main shaft in 2WD
When you shift to 4high, the gear on the front chain drive is then locked to main shaft, no grinding because both are rotating at the same RPM
Same as shifting a manual transmission without using the clutch by "RPM Matching", no grinding because RPMs of both meshing gears are the same
With auto hubs the front drive shaft is not spinning when they are unlocked
So Ford added an electric clutch in the transfer case, this was for "shift on the fly"
Auto hubs never really worked that great, and most have been changed to manual or should be, lol
i.e. 2001 to 2011 Ranger 4x4s are "shift on the fly" but NO electric clutch........................??
That's because front wheels are always locked, Live Axle, no way to unlock them, well no easy way