Likely tire, used tires can be worn unevenly. Check them for balance and roundness and try swapping front<->back and see if anything changes.

Also could be excessive caster angle, you typically add 1 degree per inch of added shackle length:
- http://www.4crawler.com/4x4/ForSale/Shackles.shtml#FAQ12

r.c.brown@ieee.org
TruckEditor@tlca.org
Project: 4Crawler
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