i have a 91 f250 has a 7.5l 460 4wd. truck has 98k miles and 3k on tranny. so this started 3 years ago when i put it in drive in runs 1st gear just fine and when it hits second its almost like the truck cant decide whether or not to lock the torque converter. like it will go from first gear about 2700rpm drop down to almost 1000rpm then shoot to about 2000 then drop down then stay locked in. it does this most of the time, but somtimes very rarely it will shift fine the tc wil lock at about 40mph and hits third at about 60mph and runs like it should. the truck sat for 3 years since it started doing this and i just now got the ambition to fix it. when i manually put the truck in second from a dead stop the truck will lightly(what i think) lock and unlock the tc once or twice then hold it locked and as stated above somtimes it will work as it should. the truck will shift fine all the time if i manually go first let it rev to about 2500 2nd rev to 2500 then drive. i have been trouble shooting alot on my own and i decided to unplug the speed sensor on the tcase i put it in second gear and it ran like it should. i put it in drive and obviously it didnt shift out of first because it couldnt register how fast i was going but didnt try locking the tc.