The other day I needed to climb a bank, so I squared in up put it in 4 lo and proceeded up the bank, but it popped out of 4lo. but it back it popped out. So I held it in , it stayed in. I was able to get it up the bank. I was thinking the linkage was out of adjustment. reason for that the lever didn't feel like it was locking in. I haven't looked into it, since I was able to do what I needed to do in 4hi using the granny gear.
actually this is the first time i've ever driven/used the truck since I traded for the truck. The truck seemed to have been somewhat taken care of, and really wouldn't see why there might be something wrong with the T-case. So I ask, hope some of you guys can suggest some possibilties.

 

I've heard of NP203s doing that after they wear. Could be linkage or the detent spring/ball gunked up.

 

Which case do you have? 203 or 205? 203's pop out when the chain is done, and often others ill do this if they are not engaged all the way.
Other problems could be that the shift linkage is hitting the body and cant engage all of the way.

 

I had a truck with a divorced 203 that would pop into 2 lo (part time converted case) It ended up being a pin on the linkage that held it in 4x4. I soaked the whole mechanism with wd40 and gave it a few wacks with a hammer and all was well. Is it a divorced T case?

 

Yes it is the divorced T case. I'm not sure which one it is. The truck is a 75 250 Highboy. How can I tell which case I have?

 

Typically each case ill have an identicication tag, and this is the easiest to help identify.
Shift lever(s) location is a decent visual way. 203 will have two levers located on the driver side of the case, and a 205 will have two rods that insert into the front of the case. The 203 will have two linkages that connect to the transfer case, and the 205 will have just one.
A 203 is a multi piece case with a range box that bolts up to the front of the chain housing. This is probably a little more difficult to see if you dont have two cases to compare.
A 203 has a huge housing over the rear output, and the 205 has a small one that simply retains the brearing and seal.

Perhaps just a picture of what you have will be an easier thing to do.