Mechanic reinstalled the L460E tranny yesterday morning. Truck drove out great for about 20 miles. On the way back to the house it started doing what it did prior to the rebuild. The truck wouldn't go into 3rd and 4th gear at 30 -35 mph. Truck had 233k miles prior to rebuild.

No service engine light or codes?

No service engine light. Everything looks normal.

Ask the mechanic for a detailed listed of the parts he put in and what he did. Your problem could be one or two of many problems. Ask him if he replaced the shift sloenoids, placer plate, and wiring harness. And I would take it back and demand satisfaction. Let me know what he replaced if you can. Also ask him if he rebuilt the valve body. Ask him for the old parts too.
SC