Vibration (spring oscillation) in rear developing a while after lowering

[rlcole321]

Well it seems I need some Brain picking form the GM truck forum, I lowered my 2018 'Rado, 4" in FRT, flip kit in the rear. After some time, maybe several months, it has developed a bad low frequency spring oscillation in the rear on the highway above 60-70 mph, I tried rotating the tires looking for a tire balance issue, but there is never a vibration in the frt end. It almost fells like a shock problem, but the shocks seem to check out fine (there the Nitro 2 shocks, I don't think there are that good anyway). So has anybody else ran into this scenario?