91 929S Low speed vibration

My car is experiencing a low speed vibration at about 15 mph. It is very low frequency, almost like a shudder or wobble, and disappears quickly. I have checked the u-joints, center bearing cushion, CV shafts, and various mountings...nothing appears loose. The trans is not slipping. Feathering the throttle or pushing it harder reduces this effect, i.e. the quicker you get past this speed the quicker it is gone. I haven't noticed anything else strange. Could this be coming from the differential? I'm not sure if this is a limited slip unit or not. I am open to any ideas.