Clunk noise

Hi, I have a 1997 GMC 1500 Sierra extended cab. I have a clunk which sounds as if it is coming from the rear axle. I thought maybe it was the transmission however it does not happen that frequently and usually makes the clunk sound on start up when in park. Any suggestions? I have 170,000K and replaced the trans. @ 134,000k

Is your transmission manual or automatic? Have you checked the universal joints on the drive shaft or were they replaced with the transmission? Maybe the driveshaft is loose and clucking at the U-joints when it starts to spin. It might also be a loose exhaust bracket, the exhaust “shudders” when the truck is started.
Sorry to answer your questions with more questions.
p.s. Do you have any dogs that jump into the bed of the truck when you start it?

Good Luck!