vibrating 95 Z71

my 95 Z71 short bed vibrates at around 45 to 60 mph .then its not so bad above and below..it has 183k on 5.7 engine.. i've rotated tires and have correct tire pressure.. any suggestionss. could it be shocks making it to that.??

First check would be tires, to see if one is badly out of balance..........

If vibration is from front end, I would guess CV axle-joint.

If vibration feels like it's in the seat, then I will guess driveshaft u-joint.

Almost definitely it is one of those things, all easy to check.