I've had similar experience with mine but I have headers and just figured it was due to all of the heat. Check your battery (load test), check your grounds, note your engine temps if you have a gauge and see if the higher temp is in fact the problem, then maybe go shop for a hi-perf starter.
Oh - one more thing - have you played with your timing lately or changed your gasoline? If your timing is too aggressive it can make it hard to start when hot.
Heat drops the conductivity level of the copper wires including the ones in the starter. Over time it increasingly reduces how well the copper conducts. Main reason heat shields are sold for shielding the starters.
Might try shorting a battery cable to the starter to see if the cable has a problem. If the starter kicks right over then maybe try replacing or rerouting any questionable wires so that they don't get soaked in the heat.
Install or clean cables. how are you grounded to engine and frame and to cab of truck? look at the big picture. ran across this several times. good luck and pretty sure your ground is shot that is for starter, engine ground and battery ground.
I'm fighting the same issue with mine.New starter,wrapped it with exhaust heat wrap and also wrapped the headers near the starter.It helped some but still have the hot start problem.I used #2 welding cable with soldered ends(before I wrapped the starter) and that also helped for shorter trips.A mini hi-torque starter will resolve it.I've used them before and they're great.This is reminding me of my Chevy engines that were prone to hot start problems.My 460 starter doesn't have a starter mounted solenoid.I thought that would prevent hot start problems.