A clunk when you press or release the accelerator can occur as load changes direction through the drivetrain. Excessive movement in driveshaft joints, axle connections, mounts, or differential components may be responsible.
Some clearance is normal, and the sound alone does not identify a failed part. Note whether the clunk also occurs when shifting into drive or reverse, and whether it is accompanied by vibration.
Contact Certified Transmission at 2112 West Broadway in Council Bluffs to have the concern checked. Finding the source is more useful than assuming the transmission or differential needs replacement.