The brief
The Jetta had been bouncing in the rear after bumps, the back end carrying on moving instead of settling, with a rougher ride and a bit of instability through turns. Those are the signs of worn rear shock absorbers. A shock absorber's job is to control how the spring moves, so the body settles after a bump instead of bouncing. It does that by pushing oil through small valves inside it, and when the seals wear and the valving tires, it loses that control. So the rear floats and bounces over bumps, the ride goes harsher because the suspension is no longer working properly, and the back end feels loose in a corner because it isn't being kept planted. Worn shocks only get worse, and they take the ride and the stability down with them.
The diagnosis
On the lift the rear shocks were checked, and bouncing each rear corner by hand confirmed both had lost their rebound damping past spec, one no better than the other. When both are tired together you do them as a pair, shocks work as a pair across an axle and a mismatch leaves the rear behaving unevenly, so it was both, a matched set.
The work
Both rear shock absorbers came off, and a matched pair of genuine VW-spec replacements went on, the upper and lower mounts torqued to the manual figures. A road test confirmed the bouncing was gone, the ride had settled, and the rear stayed planted through turns.
The outcome
No bounce over bumps, a settled ride, and the rear planted through turns. The Jetta went home with the rear suspension back to how it should ride. Shocks wear together and they take the car's composure and its stability down with them, so a fresh matched pair brought the rear back to spec.