Viscous LSD or center diff's STOP working when they get hot. That's why my 200SX has an oil cooler with electric oil pump operated by a thermal switch on the diff and a 'diff oil' warning lamp using a higher temp thermal switch on the dash.
The fluid is viscous (like glue) and so long as both output shafts are running at the same speed does nothing. If one shaft runs at a different speed the fluid "glues" plates, attached alternately to each shaft in the viscous unit, together and attempts to keep both shafts running at the same speed. The result of the shear in the "glue" is heat which thins the "glue". Without good cooling a viscous diff can and will become an open diff.
-- Peter Hill Spamtrap reply domain as per NNTP-Posting-Host in header Can of worms - what every fisherman wants. Can of worms - what every PC owner gets!