leaf spring or rearend clunk (rattle)

anyone have any ideas as too why my 98 ext sierra rattles under and too the rear. It seems to only come on after driving awhile at higher speeds but I'm not exact if that is the case or it's just hit and miss. It's more of a rattle, suspension i'm thinking as opposed to a clunk. Shifting from P to D and back no problem, that is why I don't think it's the driveline. Spring slap perhaps ? The rattle is only over uneven road or bumps, otherwise no problems. Can it be as the differential warms, the noise is more pronounced and noticable or can I rule out the dif?

Thanks all...I'm new to this and appreciate it.