3rd and 4th gear sticky

I have a 95 XJ with a standard tranny. I have problems engagin the tranny into 3rd and 4th gear but not first, second, or fifth.
Suggestions?

change lube? if no improvement, I'd wager your synchronizers are going out on that slush box. which model is it? I guess the AX15? what lube are you using? do a search about it, you may find particular lubes will eat away the bronze synchronizers and may be the source of your problem. start simple and go from there

Slush box? No, that's an AW4, he's got an AX15.

Change the oil (I like Redline MT90.) If that doesn't help, then you're going to have to get inside it. Like Steve said, the synchros are probably shot.