Car Shakes

I have a 98 chevy blazer 4X4 with about 140,000 and the car is shaking like crazy, almost like you would think it is having a seziure. I took it to someone to fix and 4 days later they still cant tell me what it is. I know it is in the rear of the car, it makes a clunking noise if u try to drive in reverse or forward. My cousion told me he thinks it may be the spider gears. He said he knows its not the CV joints any ideas? The car does need a front end alignment and a new tire on the front drivers side because of the alignment, the car is also leaking antifrezze but is not over heating any ideas?