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!!!