You can just jack up one rear wheel and count driveshaft revolutions.
If it's almost exactly 1.5 revs in to 1 rev out, it's a 3.0x,
but if you get something like 1.75 revs in, then it's likely a 3.55.
(the spider gears in the diff multiply the one- wheel- ratio x2. If you can't turn the driveshaft with
one wheel lifted, BONUS! you have a LSD!)

The winner on the M5OD is that there are lots of them out there with 1356's behind them.
So if you need driveshafts, they shouldn't be hard to find.
I hesitate to guess whether your t18 has different driveshafts- the front driveshaft from the
150 with the 300 and M5OD is the same length as the front from the 250 with 351 and ZF5...
which surprised the biscuits out of me.

t

 

If you have a tach, one could check an RPM calculator to see if you have 3.08:1 or 3.55:1. Best is to pop cover 100% as pointed out.

Do it at a Walmart parking lot, I have seen some wild stuff in those so I doubt anyone would care if you did it. Especially since under the truck and easy to not make a mess.

My 4.9 has the M5OD and lazy 3.08:1, not driven on freeway yet but cannot imagine good cruising at 70mph at 2000rpm per the calculator if AC was on and any headwind. 3.55:1 moves me up 300rpm which seems better to be more in middle of powerband.

My pal used to have a 4.9/M5OD/2.73:1 truck...OD was basically useless.