As for your second issue with the fogging windows - that most likely is due to a fresh air flap that is stuck in "recirculation" mode because the return spring holding it open to allow fresh air in the cars' interior is broken or unhinged. A common problem, but easily fixable. Read
here for more.
Even with the A/C disabled because the system needs to be recharged (and, most likely, retrofitted to R-134a in the process - check the attachment
in this post) it should behave like any car without A/C and not fog your windows.
As for the differential issues, I have no clue. The front differential is integrated into the transmission. Did you check the fluid level?
Additionally, it could be that your ABS system gives wrong information to the hydraulic clutch pack in there so it locks up when it shouldn´t.
