Yes, this is a common problem with '99 and '00. Toyota put out a technical service bulletin about this a few years ago. It would have been fixed for free if your 4Runner would still be under warranty. Do a search here for "+fuel +guage" within the past 5 years to see more about it.

Lars also has a good summary with a copy of a few of Toyota's TSB's from his web page:
http://www.larsdennert.com/4runner/fuelsender/index.htm

My fuel guage performance greatly improved after I replaced both the guage and the sender unit in the tank. Yes, it's an 18.5 gallon tank.

Jeff
2000 4Runner SR5, supercharger, rear e-locker, ARB front locker, 285/75R16 MT/R's, custom bumpers & skids
2007 Tacoma double cab 4x4 - stock