Shaking problem on Ford Windstar

Hi, I have a question about my 1998 Windstar with 116,000 miles on it, 3.0L engine and a tranny replaced in 2000. I purchased it 8 months ago used and about two months ago I noticed a slight back-n-forth shaking of the van while it goes up hill. It sometimes was accompanied by a clapping sound from the back of the van. Last week the problem became very obvious and van shakes like this quite often on any speed whether going on flat road or uphill. The clapping sound from the back (probably from exhaust pipe) became more often and obvious, especially in uphill driving.

Can this be fixed with a tune up (plugs, wires, filters change) or is it a more serious problem?