On my Dodge diesel I was having shifting issues. Shop wanted to do a bunch of work on it $$$... Called another local shop... turned out the guy was retired and the shop had been closed for a while.. but he gave me some advice... he said it was electrical. I think $20 for a new throttle positioning sensor fixed the problem.
I have it at a different time shop to use the scan machine on it and see if something else first. The transmission shop is the one this guy uses anyway so all good there. Discussing online diesel forums, they all say the same thing though.