99 dodge durango problems pcm, gauges, etc.

Where should i start?It woulndn't start. Well my durango is stuck on 3rd gear when it works, why? i took it to the mechanic and he said that the alternator wasnt getting any power due to the computer(pcm). i have replaced it with 2 computers with no sucses the 3rd computer finallly not only worked but got rid of all my previous problems except got a new a problem, none of my gagues work on the counsle and the odometer saids no bus????? What can i do?

Please help quick thanks