Just rebuilt my truck last winter, it worked before that. I had it up yesterday all wheels off the ground and put it in 4 low and only drivers side rear tire was spinning. It’s an automatic transmission with automatic hub lock( hub is sealed)

 

If you have an open rear diff, then what you see there is normal. Apply the brake a bit, and the other rear tire should start spinning as well. As for the fronts, probably the hubs just aren't locking in. Is the front driveshaft turning?

 

Is the vacuum line to the front actuator good and attached?