I have a 2007 with a 5.9 turbo diesel Cummins 48RE
It shifts kind of hard when I drive slow, and it jumps back and forth gears when I drive slow..it shifts hard when I put it into reverse, the huge clunk...no it's not the u joints cus I replaced them all.. I'm not a transmission guy but I don't have the cash to take it to one... I need help

 

See if there are any codes for a clue. (Turn the ignition key on, but not all the way to start, 3 times. Keep it there on the 3rd turn and look for codes where the mileage is displayed. i.e.: P####) It will display "done" when it's done. You can go through the cycle again if you don't catch all the codes the first time around.

I really can't give a specific answer and don't know if it would be related, but I had a '92 Cummins diesel and the "jumps back and forth gears when I drive slow" turned out to be the throttle position sensor.

 

You are going to need a rebuild. Mine was doing the second gear, third gear, second gear shuffle. My guy tore it apart and said my second band was all but gone. Fairly common issue with the 48's unfortunately.

 

Anything like my description in the below thread?

https://www.dieseltruckresource.com/...g-48re-319461/

 

Ask on these forums and find a competent rebuilder close by.
Things can happen after the build and it doesn't hurt to be local.
I had my 47re rebuilt with DTT parts. It came out well but some issues came up which necessitated a 5.5 hr drive to AZ from San Diego