IMO, here are your options:

a) Use what the manufacturer specs for the gearbox (what I'd do if it were mine).

b) Deviate and use what you're proposing.

Firm answers to b) will kick up up answers (personal opinions) that sorta fall in the category of politics, religion, evolution VS creation, global warming is caused by people VS global warming is a natural event caused by nature. You get the idea.

 

The 75W-90 will be fine. I've been running the Mobil synthetic gear oils for years with full satisfaction. If you have a limited-slip, then remember to add the "friction modifier" recommended by Ford.

 

If the gear oil you want to use meets all other requirements, I wouldn't even think twice about using a multi-weight gear oil that's 5 points lower on the bottom side, especially if it's synthetic. You'll start out with a gear oil with characteristics of a 75-weight oil only when it's cold outside. As it heats up and the viscosity modifier molecules start to unfurl, you'll get a gear oil with characteristics of up to a 90-weight oil.

My 1997 owner's manual specifies "Thermally stable rear axle lubricant, 80W90. Ford Part Number XY-80W90-QL, Ford Specification WSP-M2C197-A. Add 4 oz. of Friction Modifier EST-M2C118-A (Ford Part No. C8AZ-19B546-A) for a complete refill of Ford Traction-Lok Rear Axles."

I use Mobil 1 synthetic 75W-90 either with the GL-4 or GL-5 specification.

 

How do I know what type of axle I have in the rear of my 1998 Ranger 2WD XLT?

 

You'll find the axle code on the "Vehicle Certification Label" which is the sticker on the driver's side door.

• 84 — 3.45 non-limited slip (Ranger)
• 86 — 3.73 non-limited slip (Ranger)
• 87 — 4.10 non-limited slip (Ranger)
• 89 — 4.56 non-limited slip (Ranger)
• F6 — 3.73 limited slip (Ranger)
• F7 — 4.10 limited slip (Ranger)
• 92 — 3.08 non-limited slip (Ranger)
• R5 — 3.55 limited slip (Ranger)
• 91 — 3.27 non-limited slip (Ranger)
• 96 — 3.73 non-limited slip (Ranger)
• R6 — 3.73 limited slip (Ranger)

 

Awesome so mine was coded "86" so I have the 3.73, non-limited slip rear end.

 

I looked mine and it is coded R7. I know it is a 4.10 but I am not sure what the other specs are. Mine is making a roaring noise and have found some metal in it. I guess it is time to look at the ring and pinion gears and make a decision on whether to sell it or keep it.

 

mine is also an R7. it is a 4.10 limited slip gear box and 8.8" gears.

 

How did you determine 8.8 gears?

 

it is by the chart 8.8 axle.

from this chart: https://www.therangerstation.com/tech_library/axle_codes.shtml

 

Thanks, Hopefully I have enough information to get the parts I need to get the rear end rebuilt.