2001 Dakota constant beep while driving and rear bearings keep going out

I have 2001 dodge dakota 3.9L engine and automatic transmission.I changed my engine out 6 monthes ago because last one was shot.I used all existing wiring in truck. The headlights and all interior lights where not working. I disconected the negative battery terminal and reset the central timing module. That fixed my light problem but a beep started going off once I get up to 5 miles a hour. It does it in drive and reverse. It is the same beep that you get when your door is open but does not stop! At the same time I did the engine swap to same 3.9L engine, I had to change out my rear differential bearings because they were whinning. Now six monthes later the beep is still there and my rear differential bearings going out again!