My Jeep won't shift into overdrive. It has almost 300k miles on it and was rebuilt at 155k miles. What do I need to do?

Check for proper fluid level first. Correct any obvious leaks.
Do you have any other drivability symptoms? Like the check engine light on, etc?
If the Engine Control Module (ECM) has certain diagnostic trouble codes stored, it will prevent overdrive engagement.
Did this just happen all of a sudden?
My recommendation would be to take it to a transmission shop for evaluation and estimate.

I have a 1993 Jeep Grand Cherokee and a few years ago it would not shift into overdrive. The cruise would not work and the brake shifter interlock would not work. The Jeep Dealer told me I needed a transmission. Well a friend of mine ask me if I had checked the brake peddle switch. I checked it and it was bad. So instead of $1400 it cost me $13. Oh the jeep dealer told me he ran a scan and it showed the Transmission was bad but when my friend ran the same scan it told him the brake switch was the problem. The dealer was a crook and is now out of business.