2000 Chevy Tahoe Bad U-Joint

My Tahoe recently began making a repetative light clunking sound from under the truck. It is definatly drive train related and I can only hear it at low speed. I am suspecting my U-Joints as they are OG to the vehicle (still have the plastic gook). I cant tell how to tell if they are bad while on the truck. I did notice that the shaft that transfers power back to the the front transfer case in 4X4 has play (I can move it by hand 1/4in each way) is this normal? How do I troubleshoot my problem.
Oh and one more thing If I do replace my U joints do the stock yokes on this truck accept the spring clips on the bearing ends once the plastic is melted out? Thanks