I got an 03 gmc z71 with a 5.3 with 125k. When im on the highway or just driving, at about 55-60 the speedo starts to wiggle back and forth and then i get an abs code in my display that says service abs soon and the abs lights turn on. Then as im driving my trans starts to slip, down shifting and then up shifting. Otherwise my trans operates fine, it only happens when i get the code. I have new abs sensors when i got my wheel bearings put in. When my buddy pulled the code it said speedo sensor problem. I havent yet replaced the speedo sensor, but was wondering if anyone else has had the same problem. Thanks, Jon