Pipe dream. The AW4 always had a seperate trans computer. It moved to the drivers side at some point which is why people think it went away. Anyway, the 98 tranny will bolt right up but you'll have some issues to deal with. I'm assuming you're starting with a 90 AW4 or are you swapping this into a manual?
- The output shaft will have 23 splines vice the 90's 21-spline. Not a problem if you have the AX15 manual, btw.
- The wiring harness connectors changed in 96, so you'll need to either cut&splice or drop the pan and swap over the older sensor and solenoid wiring harness.
- In 1998, a front input shaft speed sensor was added, which you can simply ignore, but the rear output speed sensor changed from 1-pulse/rev to 4-pulse/rev. Easiest solution is to pull the tailcone off and swap over the rotor and sensor from the older unit. You could try transplanting in a 1998 trans computer, in which case you'd need to figure out the TPS signal being backwards from what the 1990 TCU expects.
- The 98 AW4 had a different 4th gear ratio. 0.705 versus 0.75. Not a big deal.