Dodge Ram 3500 6.7L I6 Diesel not shifting
 

hey everyone, new to the website and after a month and a hald of trying to figure out what is wrong I still have no idea. so basically my 1995 3500 cummins likes to shift normally until I get to highway speeds.... It will go into OD then kick out of it and sit at 2000-2500RPM (3K RPM redline) while going 55mph, then it will upshift and just downshift it'self back down.... all the time... it doesn't sit in overdrive for more than 3 minutes and i took it to a transmission shop where they put a whole new transmission and transfer case and problem still occurs, any ideas what might be going on?

You have a '95 with a 6.7 in it? That's unusual. Well, anyway, sounds like the TV cable is out of adjustment. Could also be worn out TPS.