Okay just adding to this. To help others searching when there Belarus tractor 4x4 4wd, front diff, is slipping or appears not working or not engaging, it could be the slip clutch!
As per the very helpful pic supplied by haymaker, the slip clutch may be under some guards linking the 2 drive shafts that go from the rear starboard side to the front diff. Getting the rear guard off the rear shaft off May be tricky as mine had very accessible nuts on the outside but the heads were flat head screw driver cap screws with no excess for a screwdriver (not sure if that was standard) ended up cutting them off eventually.
According to the manual (arir515, you rock!) for a similar machine (manual calls the slip clutch an intermediate bearing), the drive shaft is removed from the rear end of the slip clutch unit (I found the 14mm 1/2 drive sockets I brought to do the job were to big so maybe 3/8 would be better!)
Once drive shaft is removed from the rear of the slip clutch, there should be a large nut retained by a cotter pin. The Pin is removed and nut tightened. The manual says to tighten to between 290 and 580ft lbs or 41.2 to 82.2 kgf/meter, But this does not refer to that nut, the nut should be torqued to 57ft lbs!
Put cotter pin back, reassemble everything and see how it goes!!