Thumping/Rattling Noise Underneath

I have a 99 xj with about 85k miles. When I hit speeds over 25 mph a noise from underneath my car begins to rattle and causes the whole car to thump. I think it is a tranfer case problem cause when I put it in four high the thumping stops until I decelerate. I'm hoping it want be an expensive problem to fix. Does anyone know what this is?

have you check the u-joints?