When i hit a bump, the back end of my 2000 eldorado fishtails. Why?

On a 2006, the towing weight probably split the Sway Bar bushings. The sway bar stops the back from swaying. Could also be shocks as well or both.

Push down on the back and see if it bounces.

Could be your shocks and /or struts.

Hi, The wheel alignment could be a problem here, take it to be checked and ask them to check the "drag line" of the vehicle too.

No, a bad cat will make the engine run rough or even stall if broken up inside-then it plugs up the exhaust.
Look for loose or broken suspension links, sway bars, loose strut mounts, or other rear suspension parts. Check the front suspension, too. Suspension parts are what keeps the wheels aligned, and allow for wheel movement, independent of the car frame.
Look for loose or broken suspensio, like up and down over , , ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,......n

You need struts and a 4 wheel alignment. Sounds like a control arm in the back bent, or worn

This could be a few things that I know of. The rear air shocks compressor could be the problem. If the car was on a car lift with the wheels not flat on a ramp (like hanging) and the compressor was not shut off it can burn up the compressor for the ride sensing etc will try to level the ride but can't. or the thing just went bad. or maybe you have a bad fuse. or you developed a leak in the air bag inside the shock. or the air suppling tubes are loose or bad cracked??? but here is a start good luck mine are out and its costly to have fixed!!!!!! I'm looking to just put on a plain pair of air shocks and by passing the ride sensing etc..... due to cost. IDK but good luck!!

rear shockers gone replace both....as a pair.

TIRE PRESSURE OR BAD STEERING BOX

you mite have to change the shock on it or the swabar that mite for you

struts bearings could be worn,does make a noise that comes from the front end when you hit a bump?