Help with my suburban please

[Hank70SS]

Had 4x4 burb and the rear axle locked up. Had the fluid replaced and the rear end cleaned out and it was ok after that.

If it's in the front drive there are a few possibilities. The switch can go bad if it's electronic shift, it will do a lot of weird things. The actuator motor can also go bad, it thinks it's in the wrong position and tries to correct it. The front axle is vacuum actuated but that shouldn't be a problem here. If it fails the front axle just won't engage even though the transfer case is in 4WD. Is it electric shift or manual? Does the service 4WD light come on?

Mine is screwed up right now. It goes into 4WD ok but the light comes on after about 1/8 mile. I think the forward output shaft sensor isn't working so it thinks it's not spinning.