1984 Ford pick up with a 6.9 diesel and automatic transmission you need new ring gear Will the one from 83 to 87 ford econo van fit my truck?

It's unlikely a ring gear from a 1983-1987 Ford Econoline van will fit your 1984 Ford pickup with a 6.9L diesel and automatic transmission. While Ford used similar components across some of its lines, the rear axle ratios and consequently the ring gear dimensions varied depending on the model, engine, and options. The van's axle may be a different size and ratio than your truck's.

To be certain, you must find the axle code stamped on your differential housing. This code will tell you the specific gear ratio and allow you to find the correct replacement ring gear. A parts store (or online retailer specializing in parts) can help you determine the correct part using that code. Trying to use a ring gear from a different vehicle without verifying compatibility is risky and could lead to expensive damage.