One other option is to pull the rear driveshaft, pop the yoke (sans driveshaft) back in the rear of the transmission so you don't spray fluid everywhere and burn up your trans, and put the truck on jack stands, engage 4wd, and run it up to 70mph on the speedo. Look at the tach to see if you're turning 1750 or 2000rpm. The former is 2.94 gears, the latter 3.36.
If you try this, have someone watch the yoke to be sure it doesn't back out and cause you issues. Probably easier and definitely safer to check the diff sticker (not the door sticker) or count teeth.
If you have tow mirrors, tranny temp gauge, etc you should have the bigger axle ratio. sometimes vindecoderz can tell you What’s in it sometimes not. Mine has the 3.36 as it has the big tow package. And the aux tranny cooler thrown in...
what others said about stickers and rotating the drive shaft are spot on ways if you have nothing to work with.
whatever you do, make sure you put same axle ratio in front as back.