the rear springs have lost their ability to hold the weight. you can replace them with after market ones or a spring shop . or you could ad a 2inch block under the spring and still keep the factory shocks you can pick these up at your local parts store for about $30.00
The springs on the 1st gen 4runners are identical to the ones on the pickup truck, so when Toyota put the fiberglass shell over the back the springs couldn't support it as well. Read the wikipedia entry on 1st gen 4runners it will tell you all about it.
The 2004 Toyota 4Runner has 57.2 in. of rear shoulder room.
The 2003 Toyota 4Runner has 34.7 in. of rear leg room.
The 2007 Toyota 4Runner has 34.6 in. of rear leg room.
The 2004 Toyota 4Runner has 55.3 in. of rear hip room.
The 2003 Toyota 4Runner has 57.2 in. of rear shoulder room.
The 2014 Toyota 4Runner has 32.9 in. of rear leg room.
The 2009 Toyota 4Runner has 34.6 in. of rear leg room.
The 2003 Toyota 4Runner has 39.1 in. of rear head room.
The 2012 Toyota 4Runner has 38.6 in. of rear head room.
The 2007 Toyota 4Runner has 39.1 in. of rear head room.
The 2011 Toyota 4Runner has 38.6 in. of rear head room.
The 2014 Toyota 4Runner has 55.7 in. of rear hip room.