put it up on stands,good solid ones on concrete or asphalt, check to see if the front wheels engage. you may have to drive it and put it in 4wd before you jack it up. but once up see if the front wheel turn and if the front driveshaft turns. there are two common failure items one is the hub actuator in the front diff, if the driveshaft is turning and the wheel arent or you can stop them the actuator might be bad, you can take it out and see it work back and forth if its good. the other is i think its called a encoder motor in the transfer case and the driveshaft wont turn.
1998 Bonneville SSE
2007 Monte Carlo LT-KYB, Clarion, Polk audio, Kicker
1999 Yukon SLE-KYB, Air lift, Reese dual-cam, Tekonsha
1987 Firebird-355/th400-nothing stock