had a MASSIVE voltage drop problem (stayed about .7XX volts all the time) on my 87 Mazda B2000 but fixed it today. took out the starter, cleaned the contacts (sand paper and a wire brush will do the trick) and reinstalled it. started it up... and WOW. the thing starts like its brand new because it used to start really sluggish and slow! voltage drop now reads about .9XX on start up and drops to about 0.000 just a little FYI but i don't know if it would apply to your trucks

Keep on truckin'