99 Blazer Problems

Hello, I have a 1999 Blazer LS 4wd 4.3 engine. The engine is out of a 2000 astrovan. I'm having a couple problems with this, first after the engine was changed my gas gage reads empty and low fuel light is on. Was told to change fuel pump/sending unit, and still nothing. 2nd lately i'll be driving down the road, and engine will cut out, then smooth back out, my temp gage goes off then back on..sometimes stays off. It idles high around 2-2500 rpms, will stall while sitting at light when its in gear. After truck is warm it sometimes starts very hard, when cold starts right up. I'm getting code for knock sensor. WOuld the knock sensor be causing all of these problems? And if so, can you tell me where located so I can change it. And last my 4wd just started not working, I push the button, and you can feel in engage but front tires arn't kicking in. Any suggestions. Thanks for any help....Mike

Are all connectors connected??