All good suggestions above.
Based on your comment about feeling it in the brake pedal - Jack up the rear axle and make sure your wheels spin freely. If they have drag and don’t spin freely - it’s possible the shoes were not adjusted correctly (meaning they contact the drums when no brake pedal is applied) and they’ll warp the drums - that can feel like excessive vibration at speed.
Based on your comment about feeling it in the brake pedal - Jack up the rear axle and make sure your wheels spin freely. If they have drag and don’t spin freely - it’s possible the shoes were not adjusted correctly (meaning they contact the drums when no brake pedal is applied) and they’ll warp the drums - that can feel like excessive vibration at speed.