My new to me 2002 blazer would slip out of gear and rev to 4,000rpm before finding a gear - bad fluid or bad transmission I figured. It's got the 4L60e transmission and 170,000 miles on it. So I got a used transmission and installed it with a new filter and fluid (obviously). Finally took it for a test drive and it drops out of gear, revs to 4,000rpms before catching a gear. What are the odds I got a second transmission with the same fault?

New transmission was out of a blazer that had blown a rod through the oil pan. It makes me wonder of there's an electrical fault that mimics a bad transmission? Does anyone have any ideas before I scrap the truck? It's otherwise a really nice ride.