T-case shift problem????

I just got this 94 Cherokee and it had a bad T-case in it (it had the wrong one, a 242 vice 231 and it was in 2H and could not be shifted into 4H or L). So I replaced the T-case with one a junk yard. Now I can not get the T-case to stay engaged (in 4H, 4L or 2H), it just pops out and goes into Neutral. What is the most likely problem? Does the T-case need vacuum to shift or is it just a bad case?

How do you know it had the wrong T/case installed, and how do you know it was bad? I would imagine that this XJ has automatic...correct? On the console by the shift lever, what ranges are listed? Does it say: 2WD
4WD Lock
4WD High
Neutral
4WDlow lock

OR

2WD
4WD High
Neutral
4WD low

IF on the console by the shift lever, it is like the top example, the 242 was the correct t/case. IF it looks like the bottom example, thats for the 231. IF you switched them I would like to know how you did this. They are 2 different transfer cases. The NP231 is a part time case called command trac, with 3 operating ranges, and the NP242 is called selec trac, with 4 operating ranges. For all PRACTICLE purposes, you've just DOWNgraded your 4WD. The throw on the t/case lever will be different, and you may not have enough to engage fully, the 231. But if I was you....I'ld just replace what you had with one that works. Just so you know, the 242, is VEERRY finicky. Dirty fluid, or low fluid, can stop it from working. ...........WM