If I select drive when moving out on the highway the transmission kicks in & out steadily when it gets to the speed overdrive kicks in. The overdrive doesn't lock in --- it just continues to shift in & out constantly. This is a rebuilt transmission with 20 miles on it. I have run it only to test it in high. The rest of the time I have kept it in the next lower gear so as to not damage the trn.
transmission overdrive on 1995 Dodge Ram 3500
Go back where you had the job done. Hopefully you didn't do the job. If so I would be calling the company where you bought the tranny NOW!