rear vibration
 

2001 GM 138,000 experiencing bad rear vibration at 50 mph.....you can see front seat vibrate like crazy....feel it in the back end.
Recent control arms installed, new alignment, 4 brand new tires. Also, installed rear air shocks to lift up rear for small landscaping trailer. Before new tires, hitch, and air shocks the rear vibrated. Now, the vibration is really bad. I released some air pressure from the shocks and the car rides better but still vibrates. I only pulled the trailer once and it was empty.
Any ideas?

What I would try is moving back tires up front and recheck with a test drive. Maybe you have a bent wheel. Next, raise the back end off the ground to check the u joints. Make sure there is no play in the front and rear u joints. Replace the joints if play is observed or there is powdered rust on the u joints. While you are checking the rear joint, make sure there is no sideway play observed at the differential's pinion. Wheel bearings will growl and you never mentioned that so I am assuming it is not a wheel bearing issue.

No-Good Vibrations
 

The situation you describe is not uncommon when cars are fitted with Air Shocks, because when the shocks are inflated, the angle of the drive line is changed,
as in the Differential is tilted when the rear-end is lifted, forcing the driveshaft-to-differential angles to sharpen, which makes the vibration worse.
Letting some air out of the shocks reduced the vibration, which was already there, but made worse when the Air Shocks were inflated.
At 138,000 miles, it's a good time to replace both U-Joints as a matter of routine maintenance.
RockAuto.com for the best selection and pricing.
Let us know what you find, and Good Luck !
https://cimg9.ibsrv.net/gimg/www.mer...e3083679e3.jpg
Like No Other

Thanks guys, sounds like a plan. I added air shocks to give me some height to pull a trailer...this idea is mainly to avoid selling my car and buying a truck:) The car runs so good I dont want to sell it yet.

Myself, I would ditch the air shocks in favor of better matched springs for your loading situation. . Good upgraded springs will maintain ride height much better than stock springs.

Towing With Air Shocks
 

Regarding those Air Shocks:
You added them to allow for maintaining ride height when a trailer is to be pulled.
What we don't know is the size-type-weight of said trailer -- and those details would determine how the Air Shocks would be set up. For simplicity's sake, there's a quick and easy way to keep towing safe:
Example: Air Shocks should be "adjusted" with air pressure AFTER the trailer is hitched and loaded -- adjusted with air until the towing vehicle and the trailer are LEVEL with each other -- and when the trailer is detached, the Air Shocks should be bled down to the vehicle's NORMAL ride height. Riding around with the ass-end all jacked up is neither cool nor smart, so keep your rig at it's original ride height for best function and performance.
Consult the right-rear door jamb for correct tire pressures, and add two (2) p.s.i. to the REAR tires when trailering.
Maximum towing weight for the Grand Marquis is 2,000 lbs. -- unless fitted with the 5,000-lb. Tow Package, which I believe is now obsolete.
I had a '92 Crown Vic with the 5,000-lb. Package ( 3.23-1 Differential, ABS, Greaseable Front Suspension, Rear Air Suspension, Dual Exhaust), and it towed beautifully. No Air Shocks needed, as the Air Suspension kept everything level at all times.

https://cimg9.ibsrv.net/gimg/www.mer...d8bb92bac3.jpg
95 MERC Engine Bay

Thanks guys,95 Merc is correct, the air shocks messed up my smooth ride. I loaded up my trailer unloaded and went for a ride....no bumpy tires handle great.
1. The pics with the wheel chair ramp worked great as I run around town with my pressure washing business, here in Orlando Im super busy. Saving money for a truck this GM was my dads...paid for:) But, I am so low to the ground I cant pull in to the gas station loaded up because I bottom out....I have to be strategic.
2. I bought air shocks to give me some lift and it worked great....no more bottoming out, problem mostly solved. Started to feel some bounce in the rear....bought new 4 tires, alignment done but still a bounce.....errr.
3. Damn maybe a should have bought a truck....Biz debt free!!!!
4. Forget the truck bought a trailer 6 X 10 on heavy duty side looks great $900. cash.
5. Trailer weight 935LBs- Not going to pull full tank of water, mainly use customer water and hall my equipment.
6. 95 Merc- will inflate and deflate air as suggested.

https://cimg6.ibsrv.net/gimg/www.mer...3d35b54366.jpg
https://cimg9.ibsrv.net/gimg/www.mer...9e70d2063c.jpg
https://cimg9.ibsrv.net/gimg/www.mer...3f51bf621e.jpg

That's a nice carrier ya got on back of your Marquis, Kevin -- and what looks like a nice pair of trailers, too.
Trailering tip: When entering driveway slopes, take them at as much of an angle as you can in order to avoid
bottoming of your car and/or the tail end of the trailer.
Good luck !!
https://cimg5.ibsrv.net/gimg/www.mer...4410eddba9.jpg
Classic Style -- Relaxed Comfort -- Modern Performance.

10-4 Merc, thanks for your advice. Kevin