Mark's solution using the ALIGN command should fix the problem.
It might be easier to first draw a simple rectangle on the XY plane, then using ALIGN, select the object and press enter.
You'll be prompted for a first source point, OSNAP to the endpoint on a corner of the base of the object that you want to be the lower left corner of the object.
You'll then be prompted for a first destination point, that will be the lower LEFT corner of the rectangle you just drew.
You'll be prompted for a second source point, OSNAP to the endpoint on a corner of the base of the object that you want to be along the X_axis of the bottom of the object.
You'll then be prompted for a second destination point, that will be the lower RIGHT corner of the rectangle you just drew.
You'll be prompted for a third source point, OSNAP to the endpoint somewhere on of the base of the object that you want to be along the Y_axis of the bottom of the object.
You'll then be prompted for a second destination point, that will be the upper left corner of the rectangle you just drew.
The bottom of the object should then ALIGN with the rectangle.