The Big Bang under my 99 GC front end

Recently, my 99 GC Laredo 6cyl. began the following; At speeds over 50 MHP, there is a tremendous bang, like someone trying to beat their way thru from under the front end when decellerating. I have looked and there are no bodies or anything under there. This occurs even under light decel, like just easing the pressure on the gas pedal. To correct it, I just reapply pressure to the accelerator and it ceases.

I do not feel it thru the steering wheel or pedals. It seem to originate more from the passenger front. If I could keep to constant speed you would not hear it.

Any ideas?

I just drove it in 2nd and 1st gears and cannot replicate the noise. In or out of O/D does not affect the noise. I makes it either way.