my truck starts and idles fine but stalls out when put under a load.

I have a 1996 dodge ram 1500. It will start and idle just fine. When I put it in gear it stalls out within fifteen or twenty seconds, no sputtering or backfiring it stops like you hit a kill switch. I have replaced the engine and fuel relays just in case.
When I try to pull codes off it the check enging light blinks once quickly a loud click can be heard. I took it to a shop and they couldn't get any codes either.