5.4L F-150 dies in gear

[Ranger850]

Dad's F-150 ( '98 IIRC, 5.4L ) cranks and starts fine. Will even idle for ever, but as soon as you select a gear, it dies. We suspect a vacuum leak, but can't find it. He claims to have done a smoke test, BUT I'm not too clear on his method of said test. I'm hoping someone can point me in the direction to at least a starting point. I've never done a smoke test on any vehicle, but I'm sure I can figure it out with you guys' help.

 

[rusty ol ranger]

My 94 E150 did this and it was something with the transmission. Lock up solenoid IIRC? Not sure.

Just an avenue to explore

 

[Ranger850]

My 94 E150 did this and it was something with the transmission. Lock up solenoid IIRC? Not sure.

Just an avenue to explore

Makes sense, since he couldn't track a vacuum leak. Thanks.