1999 Ford Ranger no power to rear wheels

I have a 1999 ford ranger, 4wd manual. i just put a new transmission, clutch, slave, etc in... i completed the project just fine, everything is connected and correct... I backed it outta the garage and it was driving just fine, but then i realized that it was in four wheel drive... when i pulled over and took it out of 4wd, i had no power to the rear end when I went to take off and thus I went nowhere. changing back to 4wd from 2 it drove again... it only drives as long as the front tires are pulling it along... when i am driving i hear a clunking from the rear of the vehicle, and i havent been able to see where its coming from. is my transfer case junk or is it the rear end? thats all thats left to replace lol