1988 to 1993 Dodge-Cummins with a 5 speed have a German Getrag. 1994 and up 5 speeds have a New Venture 4500.
My 1992 have over 300K miles and no problems with the trans other then a little pin in that the shift locates in. I fixed it for $20.
I used to hear warnings about 5th gear having a lock-nut fall off. First I heard it about the Getrag and later I heard it about the NV4500. ???
I've had no problems. I was advised years ago to run the trans 1 quart over-full which I've always done. Can't say it helped anything but can say for sure it didn't hurt anything.