How many miles are on your WK? and how many miles did you drive on the new front diff fluid before your mechanic said it was low and burnt?
In regards to both diffs, your supposed to fill until the fluid starts to come out of the fill hole. And ONLY 4oz (1 bottle) per diff of the modifier is needed.
Jeep Grand Cherokee WK - Maintenance information and schedules
Try the transfer case fluid and see what happens. Im beginning to believe something isint right in your front diff OR your trans case is on its way out. Pay attention to the color of the fluid that comes out of the trans case when you drain it. Look for any kind of metal flakes or chips in the oil.
I just changed mine this past summer at 54k and the fluid was pretty clean that drained out barely and difference bewteen the old fluid and new (color wise). I hope that gives you a bearing on how your fluid SHOULD look like at your mileage. And dont mind the smell, it totally normal... youll see what i mean when your start draining it