1994 Jeep Cherokee 4.0 I6 2wd
My transmission isn't shifting into gear. My brother checked the tps and the tcu. But no luck there. What is the next step in checking things out?

Here's the thing: While driving at a steady speed ( it doesn't matter what speed) it will go from 1st into neutral ( it could be in 3rd on the hwy and still do that) without warning. What could it be? Just looking for suggestions.

 

The most common cause of shifting issues is bad shift solenoids. But you said it's dropping into neutral in both 1st and 3rd, which would be two different solenoids. I it truly shifting into neutral, or seem like it suddenly in the wrong gear? I assume you've checked the fluid level. Also check that the throttlebody-to-transmission cable is adjusted properly. It only takes a few minute to set, here's one of many youtube videos on how to do it.
If you pull the fuse for the transmission computer, or unplug in, then you should be able to shift manually with the shifter. If that's not working right, then the problem is most likely mechanical instead of the trans computer, solenoid, TPS, or speed sensors.

 

My brother came over yesterday ( family jeep mechanic) checked the TCU & TPS for correct voltage. The transmission fluid I check every day ( in neutral like your supposed to) being that I just bought this beast.
It does seam to shift into neutral. O/D is out. Tried unplugging the plug in. Still no response from the tranny. Can't shift it manually. Appears to be stuck in neutral with no R, 1-2, 3, or O/D.
Engine stays running but nothing out of the tranny.

 

When you check it are you running it through P to D and back giving the box a chance to "fill" itself or just putting it in N and checking?

You should check when the fluid is hot but if it can't drive, you won't really get it to the correct temp for that but shifting through the gears prior to N then checking instantly is a must.

 

Yep sure did. Still looking into my situation. Im just half tempted to get another tranny. But there in a nut shell If I buy another tranny, I'm buying another project. Lol. But it might be worth the hassle. Idk.