Chock one rear wheel, or stack up some boards under the tire so there's a little weight bearing on it. That will assure there's no movement as you watch the other wheel turn. A 4.10 rear end gear, which your rig probably has, will take slightly more than two turns of the driveshaft to make a single rear wheel do a full turn. The spider gears double the speed of the rear wheel if only one is turning.

That's something to remember with a vehicle that's stuck in mud or snow- - - -if a single wheel is spinning, it's moving twice the speed that the speedometer is indicating.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!