1999 Silverado is making popping and cracking noises

[jdsurge]

I don't think anyone has ever heard of this but here's what happened.  I had an oil leak, from the rear main seal I thought, and wanted to get it fixed so I took it to a local garage (that I know does very good work) and had  it repaired.  Along  with this, I also  had them put in a new fuel sensing unit  and fuel pump.  So they pulled the gas tank and changed the unit and pump and they took off both drive  shafts (4wd), pulled back the transfer case and transmission and replaced the whole rear main seal unit w/the aluminum bracket.

After that, when I take off, turn a corner, or put the truck in a bind/twist, I hear and feel a popping cracking noise like something is pushing against something,  then moves causing a loud popping noise.  I took it back to the shop but they said they could not find anything but they did hear the popping noise.

   I searched the internet but could not find anyone with this kind of issue.  I thought it might have been the gas tank rubbing against the bed moving some but the popping is coming from the front and rear.  I also thought about what they did that could have caused this problem.  I read somewhere that the drive shaft slip yoke might be dry, it does  move back and forth when taking off, stoping, or turning.  I'm going to grease the drive shaft slip yoke soon, not sure if it has a grease fitting or not, but this is a very  loud popping and cracking noise.  You can even put your foot on the brake and get  if some gas and hear it pop.  

Any suggestions would be greatly appreciated.   This truck is in excellent condition, I keep it in a garage, fix what needs to be fixed, and it looks like it is only 10 years old instead of 22 years  old.

Thanks for any help,  John