general problems of the 2001 Ford Explorer Sport Trac

Ever since I got the truck about 4 months ago I have been having a problem with the rear breaks locking up. It happens after the vehicle has been sitting up for a few hours and it's raining or there's a good but of moisture in the air. When you first get in the truck the rear breaks will lock up on you only going forward for some reason. If you apply very little pressure it does it. After about 10 times locking it up it drives fine as long as its not parked for a while. On a good hot summer day with no moister or rain it does do it at all. A few weeks ago I backed out my drive way it was raining and it did it. So I decided to pull back into my driveway and take my fiance's car to town. When I was pulling up it locked up when I trying to stop and slid into the corner of my home. No major damage was done just cracked the front bumper. I have taking it to 3 local macnics. None know how to fix. They looked at the breaks, rotors and hubs. All was fine. One macanic told me this was the 3 time he has seen this on the same exact kind of truck. I do like the truck. But I'm terrified to drive it. I thought about selling it, but now I'm in a situation where if someone gets hurt in it I would feel horrible. I sure hope someone on here can help me out with this problem.