I have a 98' F-150 4.6, 5 speed, 4x4 Off Road Package. I was wondering if anyone knows what gear ratios were offered in that package. I just always assumed that I had the 3.55's without posi. What are the other options and how can I tell?

 

Take a look at the tag on the inside of the driver's door frame. You'll see an axle code at the bottom. Here are some of the more common ones:
19=3.55 gears, non limited-slip
18=3.08 gears, non limited-slip
H9=3.55 gears, limited-slip
B6=3.73 gears, limited-slip.