So my truck will start and idle just fine but as soon as I put it in drive it stalls on me and the check engine and battery light come on. I'm thinking it's the alternator but I wanted to know what you guys thought. It's an automatic 94 with a 302 motor

 

Another member had a similar problem ahwhile back and replacing the alternator fixed his problem....shifting to drive causes the rpms to drop and and the alternator couldn't keep up.

Before you buy a new one I would pull it and have it tested at a shop. If it tests good rent a fuel pressure gauge and see if that is the culprit.

 

You are over thinking this do you have a tach if so what is it idling at it should be around 700 if its lower clean the throttle body with a white rag and a very little carb cleaner take the iac motor off the side of the throttle body and see if its clogged if so try to clean it

 

Multimeter set on 20 volts dc. Measure battery voltage across the posts. should be about 11.5 to 12.5 volts. Start the truck and check again, if the alternator is charging you should see 14 to 14.5 volts.