I have a 91 dakota 3.9 that runs fine until I put it in gear. It stalls as soon as the transmission is engaged. Doesn't matter if its hot or cold. I replaced all of the ignition components but the problem persists. I was told it had a new fuel pump when I bought it, but have not varified this. Could it be low fuel pressure?

 

I take it that you have a automatic. Anyways... is the transmission fluid at the proper level? (With the truck off and on a level surface [and parking brake applied if yours works, otherwise something blocking the wheels], shift into neutral [since you said it dies when put in gear] and start the truck. Let the truck run for a few minutes, and than, with the truck running,check the fluid level.) If the fluid level is low enough, it can cause the engine to die.
If you can find a fuel pressure tester, fuel pressure should be around 15psi (since your motor is throttle body injected).