Flip the rear shackles. It will give you about a 2 inch drop, won't affect load capacity, and only costs what you'd pay for bolts to put it back together.
Steps:
1. Drill/cut rivets holding stock shackles in place
2. Flip shackle over
3. Re attach shackles to frame in inverted position, using grade 8 hardware
**Disclaimer**
rear shackles love to rust out on these trucks. If yours are bad, just replace them entirely, and skip to step 3