First, double check the seat to seat frame bolts. My XJ ones backed out a few times before I got enough sense to but locking washers on them.
Second, Check the plastic rollers on the seat adjusters....could be broken.
Do you by chance have after market power seats?
Current Jeeps: 1998 Jeep XJ- SOLD! 2014 Granite JKU Rubicon X 730n/ Dual Top/ 6SP Manual 2004 Ford E350 12 Passenger Van, 6.0 Diesel with Quigley 4x4- Cold Weather Package
If you have checked all the bolts and they are all tight then you might have a broken the internal frame of the seat. Makes things kinda unsafe if you get into an accident. Get it checked by the dealer if its under warranty. If it isnt then maybe its time to look at a seat upgrade.
Yeah the seat frame could be cracked along a weld. XJ's were notorious for the seat frame cracking a rocking back and forth. Many guys just re-welded them. I bought a new Jeep police spec seat frame that was HD and built to withstand some major force.
My guess is Jeep makes similar police spec components for the JK. If you need to buy a seat frame, then I would ask the parts guy to check PN's for police spec component.
__________________ 2013 Wrangler JKUR10A 3.6L DOHC - MDH 032515
:: 2015 KTM 500 EXC 510cc - CLEAN suspension
:: 2002 Chevrolet G3500 LWB 8.1L - motovan
:: 1991 Cherokee Laredo 4.0L HO - resto in-process
:: 2010 Cherokee SRT8 6.1L Hemi - sold
:: 1965 Plymouth Sedan 562ci Hemi - sold