On my 1991 f150 xlt lariat 300, when I shift into reverse or drive the truck wants to stall and sometimes does? Could it be vacuum?

 

Pull your codes as they might lead you in the right direction. If you still have the original small diameter plastic vac lines then it's a good possibility. Also could be the alt getting weak, transmission issue, TPS, etc...

This is one of those situations where you are going to have to start eliminating things. Start with the easiest to test.
Test vacuum from each of the lines coming off the intake tree.
Test tps for .9-.97v closed and 5v WOT.
Test output of the alt.

 

Mine did this when it was lowing tranny fluid

 

Good idea

 

Mine also dose this turns out my vac line was extremely lose

 

I had the same problem with my 91 it was a old tranny filter an low fluid combo. I replaced the filter an gasket along with fluid has not done it since.