I can relate to the OP somewhat. The main hurdle for me was to sacrifice the pay to get to 1500 hours. I drive trucks also but not for UPS. I do LTL and put in same amount of hours as you do. Best option is CFI if you have one and/or. Banner towing depending on where you live.
If you could somehow survive a temporary paycut, I highly recommend doing 1-2 season of aerial survey. You will have your hours by than. Survey season is typically from oct-may which means summer months you are off. You can always go drive trucks or banner tow.
Can you ask UPS to give you a leave of absence?