You have to follow the power. If your on the gas and the truck isn't moving, but the engine is rev'ing, your power is being lost between the engine and the ground.
1) Watch the spur/CB. if they aren't spinning, then your clutch is gone or your flywheel is spinning on the crank.
If ok, move on.
2)Check the shaft the spur is on. If the spur is spinning, but the shaft is not, your slipper is either loose or fried. Tighten or rebuild.
If ok, move on.
3) Check the center axles. If they are both not spinning, then your issue is in your trans or the pins fell out that hold the axle cups onto the trans input shaft.
If ok, move on.
4) If the center axles are spinning, but the wheel axles are not, then either your diffs are blown (badly), your pins broke inside the diffs or the pins came off of one of your center cups on both ends.
If ok, move on.
5) If the wheel axles are spinning but the wheels are not, then you blew a wheel hex or axle pin.
If it's inside the trans, it could be a fist full of things, like not being in gear properly, having gears with the teeth ground off inside, having a blown OWB in the two speed, a sheared pin for one or more of the gears, a pin that fell out inside for one of the gears... Also, check all dogbones for outer and center axles. Should be pins that ride in the slots of the cups, they wear down over time and can eventually break off.
That's the general process of elimination of drive train failure in most rigs.