sounds like worn out shocks/springs.
Mine did the same when I bought it, put new shocks in it along with adjustable lower control arms. Now whenever a different tire size is put on I can move the rear end of the car up or down to adjust it accordingly to look good.
'04 Roush F150 ( totalled while parked
'89 Gt Hatchback H/C/I, 1.6 rockers, 75mm maf, 24lb, 70mm tb, cai, udp, full exhaust, msd ignition, 373s, Eibach Springs, Lakewood 50/50 rears, Adj Rear Control arms, A/C Delete, Smog Delete, EGR Delete,