Check the parts in the manual, most likely you have a collar with set screws underneath the playfield to loosen and remove the diverter shaft.

LTG

It's been ages but I'm pretty sure you can loosen the diverter's assembly or arm from under the playfield just loosen the set screw(s) with an allen wrench. Then remove the top plastic cover from the ramp, lift out the diverter and the ramp should come out easily after that.

From pinwiki:
http://www.pinwiki.com/wiki/index.php?title=Indiana_Jones:_The_Pinball_Adventure_Shop_Guide

Locate the solenoid that energizes the right ramp diverter. It's under the right ramp (under the playfield). It's a yellow coil number FL11753-1. Out of this coil is a metal shaft with a spring. This shaft is tied with another little spring to a part that 'screws' to the bottom of the ramp diverter piece. Locate and loosen the set screw in the side of this. It's a small allen (hex) screw which should be 'facing' the left side of the pinball machine. Unscrew this set screw so that we will be able to pull up on the metal ramp diverter later on and pull it out. (When we do so, this piece will fall down in the cabinet).
Unscrew and remove the plastic piece which screws into the right ramp which covers the ramp diverter piece.
Pull up on the metal ramp diverter piece. Keep the washers with this piece, we don't want to misplace them. There should be two washers under the diverter's 'arm'.
The part under the playfield should now fall down, be sure not to lose it when we re-assemble it. There should be a metal shaft that went into the coil. It should be attached by a spring to a piece that 'clamps' to the end of the diverter. The second part that fell is a spring that went around the metal shaft as it went into the coil.
Remove the right ramp assembly. Three screws at the top, two mini phillips screws at the bottom and two regular screws at the bottom sides.