you need to find out exactly what kind of trans is in your 90 EB, if its the computer monitored e4od then your ecm will store trouble codes because the trans in the 85 does not have the plug ins to communicate with your ecm, however more likley you will have the aod trans which has nothing computer related like the trans in the 85 and would be a direct replacement.
you may want to be sure you want the trans in the 85 and identify what it is, cause after going from a 3speed to the overdrive i would not go back,
PS check your engines aswell, broncos in the 80s and 90's primarily came with 4.9, 5.0, and 5.8 engines, the 5.4 engine is the new generation that was introduced in 1997,