I was about to say what Plowking said to do.
( try disconnecting your reverse pickup wire on trans and put the truck in reverse see what happens)
Weird thing about that is that my tach on my 81 is completely messed up and reads 2250 RPM at idle (WHEN it is not) and when I put it in reverse, it reads normal.. Very weird.. Im sure thats got to be pretty annoying.
