Since you said you put different tires and rims on it from another car (assuming they were good), it seems warped rotors are the most likely culprit.
Changing out the rims and tires should have netted you SOME kind of change.
A warped rotor would drag once every revolution and produce a pulsating sensation and if severe enough, could cause your steering wheel to move back and forth.
Alignment is another possibility...but it would have to be off pretty badly.
I would suggest an ABS check but the chance of that causing it is really remote.
My bet is on warped rotors(s).
1999 Grand Caravan SE
2006 Caravan SXT