1997 GMC K1500 Z71 140,000 miles V8 5.7 with a vibration

Start with the obviuos ( I spent 13 years in the tire, front end business) Check your Lug nuts on the front wheels since you just had rotors replaced. The second thing I would look at is your Driveline here is where you would most likly feel the vibrations under your seat. Pull it and have the balanced check. Last thing I can think of. Did they rotate your tires while getting rotors put on? Did they Balance them? Rear wheels could be out of balance. Hope that can help those are the three most common things I have dealt with. If you don't feel it in the steering most likly its Not the front end or alignment.

How many miles are on it?
Have you checked your U-joints?
Also, if you have a good amount of miles, you might check the tranny and all the other mounts that hold the drive train in. A loose bolt could result in vibration.
I had a lifted Blazer and I could tell you from how the vibration felt that I had a U-joint going.

Good Luck