Technically, the topic is Grand Vitara rear brakes, so it's not really a hijack
Does your truck have anti-lock brakes?
If you don't, there will be an LSPV (Load Sensing Proportioning Valve) on the right rear frame rail that may be acting up - what it does is limit the pressure going to the rear brakes to prevent them from locking, and it does this based on the "load" in the back of the truck - the greater the "load", the more pressure the valve allows.
The problem is the LSPV doesn't actually sense the load, it senses the height of the back of the truck and assumes that if it's lower, it has more of a load in it - so with that 2" spring lift you have, if you didn't adjust the LSPV to compensate, it sees the lift as no load at all, and reduces the rear brake effort to almost nothing.