Ford did have 2 types of F250's in that year. The best way to tell them apart (that I have found) is either by the weight rating, or the rear axle type. Check the weight rating for the front axle on the sticker on the door post. If the rating is 3800, that is considered a Heavy Duty. You can also look at the rear axle. The Heavy Dutys of that time frame got full float axles.

HTH

Roger Lane
Test Analyst
Sr. Automated Test Engineer

PS. I will check the weight rating on my '89 later and let you know what it is. Mine is considered the regular duty F250.