i have noticed the past few months my steering wheel shakes at around 55mph and progressively gets worse. at 85 mph if i have a drink in the cup holder its bouncing like crazy. tire are new in front about 2 weeks old stock tires size, tire pressure good. 03 2wd cummins truck has 72k miles on it. back wheels are 31" dune grapplers about a year old. truck also has 1.5" coil spacer if that makes any difference.
Just wondering what could be the problem? thanks