Are they on angles/noseup/down when this happens, or is it sporatic? Could be driving the tank unit crazy.

If it does it while doing everyday driving, here's the other take.

They might want to either get with the dealer and their VINs to check if there's a TSB out there for fuel gauge inaccuracy; I've got one for my 99 Runner. The literature in the TSB, if there is one, will list VIN, and tell you if the gauge needs replaced, or tank unit. In my case, it's both.

They can also do the ol' $10 download at www.techinfo.toyota.com and snag any info they want.

I went to the dealer, and got the mechanics instructions right down to the replacement part numbers for mine; simply had to ask.

Go that route first.