I am 99% sure the Aerostar used the A4LD in 91 (I KNOW they did in 92). The 94 EX still used the A4LD, and I do not think there were any changes in the PCM control aspects. There WERE significant improvements to the A4LD in 93, but in general, Yes, I think the 91 Aerostar A4LD should fit in your 94 Ex.
Happy Exploring
Chris