Q: engine stars and runs smooth. but stalls when in gear. on 1994 Toyota Pickup

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the check engine light is off. but as soon as I put the engine in gear, stalls and won't go. The gas filter and gas pump had been replaced. The trottle body and air flow sensor had been repleaced. What's wrong with my pick up?
(1) Answer
Automatic or Manual transmission?

If automatic, could be a transmission issue as it happens as soon as you put it in gear. Can you keep it running shifting it into gear brake torquing it?