Tranny or Worn Gear???

I have an 00 eclipse GT (automatic).  When I hit between 35 -- 45 mph I get a serious shaking in the front end.  I've noticed that this only occurs while in 4th gear.  I've had the alignment checked; also checked the tires and they don't seem to have any warps in them.  I was told that I might have a soft code on the ecu.  Does anyone know how to pull the codes off?  I've found a reference on what the codes mean, but I need to pull them off the ecu.

any help would be greatly appreciated; also if anyone has had this problem please tell me what I can look at to fix it.

Thanks,

el tigre1