98 Cummins, 4 Speed Automatic. 67K original.
I bought this truck thinking that I was getting a cream puff and for the most part it is, except for some nagging issues with the tranny. To begin with, it had a bad torque convertor. So I had that replaced and while the tranny was out, they serviced the tranny, new filter and amsoil synthetic fluid, replaced the seal between the tranny and transfer case, $1,800.00. Now the tranny will not shift between 3rd and 4th, you can feel it trying but it just doesn't get there. I have read somewhere on this forum that there may be a service bulletin dealing with this problem. The tranny guy initially said there wasn't any connection between my current problem and what they did. Perhaps not but it sure seems like a strange coincidence. I noticed the current shifting problem within a couple hundred miles after the tranny work. It could have been there right after they worked on it and I just didn't notice it. HELP please.

 

I had a similar problem with my 98 tranny. Believe it or not it turned out to be dirty BATTERY TERMINALS !!

These tranny's are electric, so having dirty terminals screws up the signals or something. Once I cleaned the terminals up the trans shifted again like new. Keep in mind that my terminals weren't that dirty. You could see a tiny bit of corrosion, but it wasn't like caked with it. I cleaned all 4 terminals and also the groung where it meets the block and once again there was joy and love. I sprayed some purple Permatex stuff on everything to help preven further corrosion.

Give it a shot. It's free and you certanly won't be hurting anything by doing it.

Good luck !!