You might save yourself some work by just jacking up one side of the truck.....if it's an open (not limited slip) you should be able to spin the raised wheel. Make a chalk mark on the tire, make a mark on the diff yoke. Get a helper to spin the tire 10 revolutions while you count the revolutions of the yoke. Record the yoke revolutions and multiply by 2 and divide that number by 10. That should be the first 2 numbers of your R&P ratio.
So why not just spin the tire once? With just one spin you will be able to figure the 1st number of the ratio but will have to eyeball what percentage of a revolution the yoke made. Is it a 3.00, 3.28, 3.38, 3.42, 3.50, 3.73 or a 3.90? By spining it 10 times you get the first 2 numbers of the ratio. Example: Ten tire revs =17 yoke revs times 2 (because only one wheel is turning) divided by 10 gives you 3.4.....more than likely a 3.42 R&P.
Granted, pulling the cover is going to give you numbers to work with....but it's also going to create additional work/mess and you'd better know which numbers to use to make your calculations. This process makes it easy in a junkyard to determine the R&P ratios without having to grapple in the dirt!!!!! Hope this helps.
Dave
