This could be a couple things. If the clutch is not completely disengaging, it will make shifting into all gears difficult.
Try this:
Shift into first gear and 4 low on flat ground. Take the parking brake off. Hold the clutch pedal all the way down and rev the engine. The jeep should not move. If it does, there's an issue with disengaging the clutch. If not, it's probably the transmission going bad.
Next step is to change the fluid in the transmission. We generally recommend Redline MTL. It may take a few days for it to improve, if it does.
If it still does not improve, you may need a new transmission. With the NV3550, there are no new units left. You can buy a remanufactured unit for around $2000. A better option is an AX-15 retrofit. The AX-15 was offered in Jeeps up through 1999. And it's actually back in production, so you can buy a brand new AX-15 for about $1600. There's a couple parts you have to swap, but for the most part it's almost a perfect swap.
Here's one source:
https://www.advanceadapters.com/products/26-ax15--aisin-ax15-5-speed-transmission-new/