Sorry dude, your 1.5" is about the highest you should go without a suspension lift.
You can probably add another inch with shackles, but realistically, springs are about in order.
Easy options can be had, spend $500 on a lift that includes springs and all you need to lift it 4" then buy bigger tires....
other options can net you 2" with Waggy springs,. but you will also be stretching it 2" per axle... so drive-shaft work is necessary.
What is your reason for not going with a suspension lift?