I put in another computer.................

and my '89 will still not charge the battery when it is going down the road. Also have a new alternator and starter. Charges just fine when sitting still, out of gear (4WD - manual).
I just noticed that the 4WD light stays on all the time - even when not in 4WD. Could there be a short in that area somewhere? Why else would it charge only when in gear?
I thought it was the voltage regulator (which is located in the computer I am told) so that was why I replaced the computer. I am getting really tired of having to leave the truck running for 15-20 minutes when I get somewhere. But at this point I have no choice. Please - Any ideas at all would at least give me somewhere else to look.