Toyota Repair: 4Runner brakes engange in 4wd rear gear in slippery conditions, rear disc brakes, 4runner sr5

Question
I have a 1999 4Runner, SR5.  When I am in snowy slippery conditions, with the 4wd engaged (I always use the Hi as opposed to the Lo option) and put the car in the back gear, it appears as if the brakes engage. I can hear sqeeling of what sound like the brakes and the car wont go anywhere.  It is as if there is a big hurdle behind the car (presumably because the brakes are mostly locked) and then eventually one tire starts spinning without the car going anywhere (the other tire presumably stuck).   What gives, is the antilock brake system going bunkers here, is there some brake adjustment off ?
Thanks,
Oli

Answer
If the ABS had a problem it would turn the warning light on the dash on. It seems there is a problem with the rear brakes, the drums should be removed for inspection, there may be an axle seal leaking or possibly a leaking wheel cylinder, does it have rear disc brakes or drum?