I have a 2006 had 2500 w/ an 8.1 liter engine/ 6 speed Alison transmission. I’m having an issue with my transmission I can start it up and pull forward 6feet or so it will do well in reverse .

 

I wish I could help you but I'm no expert like I am with other things like engines and A/C systems. Allison's are said to be one of the
best auto trannys there are, but I don't enough to help you. I hope someone else can here can help you.

 

Have you checked the fluid level and condition?

Will it drive further if you select manual 1 or 2?

Is the gear shift indicator (on the dash) flashing when you select drive?