1999 chevy silverado getting vibration at pedal and steering wheel 45-on up. truck sounds different. new shocks, tires balanced. goes away when off the gas pedal check engine light came on

Harmonic balancer.

possible tire balance out of whack. cheap quick fix get your tires balanced.

How about motor mounts.

If the tires are round once checked on a balance machine to be true. Next check the driveshaft is a weight missing, dings or bent, u-joints loose?

Then check the rear ring

Your front tires could be out of balance, or out of the round. Have it balanced.
Shock absorber and /or suspension components worn or damaged.
Lose or worn out steering parts.
Doe's do it when you braking.

bent alloy, bent steel rim, broken belting inside of tire(caused from hitting a good size hole). lug nuts not torqued to correct ft lbs, outer or inner tie rods, toe/ cmber out of adjustment. Are the tires wearing? If not, forget the camber/toe

I just had the same issue with a 1999 2500HD and the vibration got worse above 50 mph. Problem was resolved with a new driveshaft. Old one (aluminum) was badly out of true and the problem didn't really occur until I had the U joints changed - then bent driveshaft vibration was noticeable. New driveshaft resolved issue.

If you get a bad vibration in your steering wheel at low speeds without braking it is either a bad tire or a bad rim. If you get a vibration at speeds around 45-50+ and goes away you probably have a balance issue. If the vibration only occurs while braking then your rotors are warped. If low speeds it shakes i'd take it to a shop to check your tires.

try a transmission FLUSH. I did it on my 98 1500 and it helped

Have the balance checked on the tires.