Ford F150 Forums - Ford F-Series Truck Community. - 2001 F150 stalling when put in drive or coming to stop

2001 Ford F-150 5.4L V8 Lariat Screw. 251,000 miles.
Okay, so my engine stalls when I put it in drive or when coming to a stop. If I switch into N or P it returns to normal idle. It seems like not getting right air/fuel mixture when load is put on engine, but also could be transmission. No OBDII codes to help with troubleshooting, FORSCAN doesn’t tell me much either.
Also have developed a bad coolant leak so today started stripping truck down to take out intake manifold to look for cracks or vacuum leaks because coolant is sitting on top of engine.
Got to Thermostat and it’s never been changed, was almost like welded in and had to use a flathead to forcefully pry it out. It’s supposed to open and close easily right? Not a good sign?
Also took alternator to get it tested and tested bad at 2 stores and good at one store. Never thought of it being an alternator problem because anytime I ever checked with a voltage meter battery always read good and charged. Here’s a pic of one of the tests if anybody could tell me what this means.

https://cimg1.ibsrv.net/gimg/www.f15...e09428243.jpeg