My sonoma vibrates bad when reaching 60 mph and slowing down until under 60 mph, what could this be?

SOURCE: 2000 GMC Sonoma Vibrates at 25 and 45 mph

Have your tires checked, you probably have a shifted belt in one or more of the tires.

SOURCE: Vibration on fuel pedal of 1999 suburvan

This may be caused by bad motor mounts. Lift your hood and then start the engine. Put it in reverse and hold the brake. Push on the gas peddle and watch the engine. If it trys to roll over they may be bad. Be careful.

SOURCE: Front vibration in 1998 GMC Sonoma extended cab

since you are getting your car aligned make shure that they check the constant velocity joints.....

when these are bad...what you get is a vibration in one o both sides, also you might hear a tapping sound when steering.

SOURCE: vibration between 55 and 65 mph

I am a minor shade tree mechanic here so this is just a guess. It might be a front brake rotor, a break pad caliper that has a sticky hydraulic piston, front wheel bearing, or something else. Look at the pad and rotor wear for it possibly being a break system issue. Not sure, if you do not have a solid front axle look at the inner part of the wheel under the vehicle where the ball mates into the socket for any wet fluids or unusual gunk or build up around the axle and break calipers for a wheel bearing problem. Jack up the front end and make sure you chalk the rear tires, vehicle is in PARK and the steering wheel is unlocked as to where the steering wheel can be turned if needed. After vehicles front tires are off the ground try to pivot the front wheels to point in a different direction left turn or right turn. This can be tried by physically grabbing the tire trying to move the tire to point left or right. If the front tires are able to be pointed in a different direction without the aid of the steering wheel, then it could be a wheel bearing issue or a more serious issue. Hope you can make sense of these instructions, and hope this helps you find and fix any and all existing problems you are having with your vehicle.