Older Land Cruisers that had manual front locking hubs never required a CDL, it was always on... so to speak. When you lock the hubs on dry pavement it would create a lot of stress on the transfer case, not recommended. On surfaces with less traction the transfer case has a chance to "unload". With full time 4WD, they "viscous clamp" the driveshafts, which allow some "slippage". This keeps the T/C from binding up. When you press the CDL button, it locks the T/C. You now get 100% power to both driveshafts. Not recommended when driving on high traction surfaces.
HTH