[Stocker]

$15 is too much for a quart of synthetic gear oil -- look for a different brand, check at other stores, or just go with dino. Probably worth changing since you recently got the truck and don't know when it was last changed.

As to gear ratio, this is the method I use -- I'll quote board member raycow from a previous thread.:

Re: How do you calculate your gear ratio?
You don't have to take off the cover unless you really enjoy draining the oil, getting yourself greasy, scraping off the old gasket, etc.

Put matching marks on the pinion yoke and differential housing. Jack ONE rear wheel off the ground, put the transmission in neutral, and the parking brake off. Turn the wheel exactly TWO revolutions and count how many times the driveshaft turns, estimating any fractional turn as closely as possible. That number is your axle ratio.

If the wheel is difficult or impossible to turn, you may have posi. In that case, jack both rear wheels off the ground and turn either of them only ONE revolution. Count the driveshaft turns the same way as for an open rear.

Ray