You can permanantly lock the passenger side front axle on a disco front end with no issue. You then have a front end just like later XJ's/TJ's, etc without the disco. No effect on street driving, it's still 2wd but both front axles now spin all the time. The T-case is still in 2wd and not applying power to the front end. The easy way is to remove the shift motor, take it apart, and shim the fork so it keeps the collar in locked position. The other way is to get rid of the disco motor and just use a later model one piece shaft. You need to remove the carrier and install a special seal in the axle tube. The part # is floating on this board somewhere.
The handle on the floor controls the range of the tcase. When you put in 4WD, it activates a vacuum switch on the back of the case that moves the shift fork on the disconnect to lock the front axle. If you permanantly lock the axle or put in a one piece shaft, then you trash all the vaccuum lines and plug it at the source.