Bad tranmission or rear end? AstroVan

I had just bought an 96 astro van, rwd. i drove it home about 45mins away with no problems. next day, i got it on the road and drove it about 30 miles, picked up a buddy, drove like 4 blocks when i heard a loud pop from the back. i thought it might have been the e brakes. (since earlier in the day i had used them but forgot to take them off, i had backed up and couldnt pull off. released them and it moved fine and thats where i started my story.) but i tried to make a right turn and i felt a clunking in the rear end, and it was “jerky” until i straightend the wheel. it drove another block when i tried to turn left and it didnt move. it just revved with a sort of grinding sound. i placed it in reverse and it jerked beck a couple inches, to a foot. back into foward position and nothing. we pushed it off the road. when the tow guy came, we placed it in neutral, hooked up the cable, and as he pulled the van back end first (or in reverse while in neutral) it jerked every 6 inches or so. but push it foward its smooth(in neutral). now is my tranny shot or the rear end? Please help! i bought a rear end to put in, as i have it up on the jack stands i checked to see it the tires moved, and they do, just no power when i accelorate. i dont wanna put in the rear end if its the tranny. i wanny try to get credit towards the tranny if needed.