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Basic part time 4WD system.
1) Does the transfer case engage? Does it send power to the front driveshaft when in 4H? The most common cause of the t-case not engaging is a dirty shift motor.
2) Assuming the differential is good, so that a spinning driveshaft also spins the axleshafts, do the wheels get power from the axleshafts? If the axleshafts spin without spinning the wheels, then there's a problem in the hubs. Do you have manual or auto hubs? If manual, have you locked them?
And if auto hubs, take the wheel off and look behind that little cap, see if anything's broken off, like the gear looking thing. Sorry I'm not big with the technical terms.