I have a 2000 Blazer ZR2. The truck would vibrate (shake) really bad at approx 70 miles per hour. As the truck gets up to speed there is a slight jitter or thump as the truck goes from 20-30 miles per hour. My BFG all terrain tires were worn (above lincoln head on penny). I wasn't sure if a rim was out of balance or something in the drivetrain was causing an out of balance issue? Last week I had the tires replaced and the problem was barely noticible, even at 70 mph on highway. Now after a week of driving, the problem has reverted to its original state. As a hunch, I removed my custom hypertech transmission shift firmness and alteration program and it is now stock. No difference! From 2000-2005 I had a meyers trimline snowplow on the truck. Not sure if the transmission would cause this vibration (shake)? Any ideas? I do recall having this problem start last year after having brake rotors replaced and new pads installed. Could that cause this type of problem?

Thanks for your help!

Where do you feel the vibration? Seat, steering wheel, floor?

When my 2000 Blazer had those same symptoms it turned out to be bad ball joints, a common Blazer problem. How many miles on the vehicle? Ever replaced the ball joints? Check them for play -- the lowers usually go first, but given the amount of work involved, it makes sense to replace all 4 if you are doing the job. If that is your problem, spend the extra money to get oem or Moog parts unless you want to risk replacing them again in a year. There are some really crappy aftermarket ball joints out there.

Always check for worn/lose parts and bent rims or slipped belts on tires.

But the major cause of viberation or shake at 60-70 MPH is tire balance.