86 c-10 2wd p/u. Trans will not downshift to 1st gear when at a full stop (starts off in 2nd) you can pull it into 1st and it will work. Will shift from 1st to 2nd fine but will not shift into 3rd. Its like having a stall when you let off of the gas. It is still in 2nd and will not shift into high. This happened all at once no warning at all. Ck vac to mod valve and ck for fluid in mod line and there is none. Fluid level is good.