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It all depends on where the leak(s) is coming from. On the transfer case, I paid about $85 for a sensor that the shop said was leaking. From the description, I thought the sensor was cracked, but when I examined the old part, it looked fine. The o-ring had gone bad. Anyway, the new sensor leaked after about a year. I took it off, replaced the o-ring (about 20 cents at the local hardware store) and it hasn't leaked for more than two years. If the transfer case is leaking between the case mating surfaces, then there will be a lot more labor involved.
On the differential, I've had a pinion seal replaced for somewhere around $150-$200. I would imagine a leaking cover or speed sensor would be cheaper.
The mating surfaces between the transfer case and transmission uses a regular paper type gasket. If your case is leaking between the TC case halves, there is no gasket per se, only a thin film of sealant applied at the time the case halves are mated. Usually once bolted up, neither gasket nor sealant develop leaks. It is as Mikeman says, usually the O-ring on one of the sensors or from around the speedometer drive. Replacing the paper gasket, involves taking off the transfer case and after having done this, I would not recommend trying unless you have access to a shop hoist. You need a lot of room underneath to work.
One last note of caution, the bolts that hold the TC halves together are a self tapping style that cut into the soft magnesium of the TC. Do not overtighten these, trying to stop the leak, or you run the risk of stripping the case threads.
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