This sounds terribly familiar. I was having random weirdness starting the trooper over the course of a few weeks, everything from not starting to hard starting to only starting with a jump. Eventually I got in, turned the key, and nothing, just the click from the relay. I replaced the relay fuse (blown) the relay (just in case), cleaned the a/t range sensor, tested the ignition switch, all nothing. Turns out it was indeed the starter, fired right up after replacement.
Its super-cramped under there.
If you decide to do the job yourself and don't relish taking the exhaust, etc off, try a trick I found over at planet isuzoo from a rodeo guy: undo the wiring and unbolt the starter from the vehicle, then using a 10mm wrench, remove the solenoid from the starter. pull that from the truck, then use an 8mm wrench to remove the cylinderical half of the starter and pull from the truck. Theres a cap over the 3 gears on the remaining piece, remiove that (pull) then maneuver the remaining part from the truck.
Install is exactly the opposite- take your new starter apart (Carefully!), push each piece in and reassemble one at a time. Just be careful not to lose the little gear bearing things, glue em down with grease or something. Then bolt the reassembled unit to the truck.
Total time (including messing around with the exhaust before giving in and trying this): 2.5 hours.
