How do I make this work?

[C B]

We currently own (free and clear), (2) 1-1/4 acre adjoining parcels in a growing, upscale, residential, neighborhood 20 miles north of Phoenix. Several million dollar homes nearby. I estimate the value of both together around 300-400,000. it was worth quite a bit more 12 mo. ago. The original plan was to build our dream home. Now the problem, We live in a small home (1348 sq. ft.)w/3 growing children and need more space NOW. We would like to trade this property for a home worth 300-500,000 and perhaps make monthly mortgage payments on any difference. We have PERFECT credit, not a single mark in well over 15 years.
Here's the kicker. Our debt to income ratio is about as high as it can go. Our income is not very high. We own a home now that we would like to keep as a rental until the market recoups. How can we do something like this w/o having to qualify for a mortgage?