Could be the Rubber Fuel tank grommets and pads are bad.

My squeak turned out to be my fuel tank.  The tank is supported by four
rubber grommets on the two brackets that hold up the tank.  There are 6
rubber pads, 3 on each side that are between the tank edge and the frame.  
The four grommets were squashed and that allowed the tank to move about
slightly.  The 6 pads were also dry out.  So when I went over a bump or rough
road: SQUEAK, SQUEAK, SQUEAK.... I greased my upper and lower bushings,
nothing, thought it was my spare tire moving about, bent the upper rods on
the tire carrier to hold the tire tighter nothing...  As it got worse,
shutting the doors caused the squeak, this lead me to the fuel tank.

I put in new grommets and pads and put some Teflon grease on the pads and now
no more squeak from the front.

Robert