engine stall

i have a 96 chevy silverado 4x4. when i start the engine and it idles down, it will stall and just about die. when i rev the engine up and let off the gas, the same thing happens, it will idle way down and just about die. when this problem started, the transmission also began shifting harshly, it bangs into every gear. I changed out the trans filter and replaced the 0 ring seal. no improvement. it seems like it might be a vacuum leak, but i can't find one. any help would be appreciated.

Any check engine lights on?
A good scanner should point to the problem.
Post back any code no.
MT