I would first be checking the rear shocks as the source of your "conk;.

 

A common source of clonking sounds from the rear is deterioration of the upper shock absorber foam rubber bushings, Parts 13 & 14 in this diagram:





Compression and deterioration of the large foam rubber "donut" spring isolator bushings, Part 5, is a common cause of loss of ride height.

Cheers,

Don

 

These could also be another source of a soft thud. If you aren’t familiar with these, these are the muffler mounts located above the differential. You’d have to get under the rear of the car and move up and down the exhaust to see if there’s excessive play from those bushings.