I am having trouble with my 4th gear. I put it in D and drive normally and when I get to the 4th gear switch it's like I threw it into Neutral. It won't go..it won't engage the 4th gear. I have to push the O/D off button so I can stay in 3rd and it won't screw up. what's wrong you think?

how does the OD clutch go bad by the way? I mean, I can't exactly shift aggresively into 4th gear lol. I can still get around with my truck.... today I had to drive like an hour to my parents house and had to stay in 3rd gear.. which I found out can get at least to 85 MPH in my truck. I have the 3.55 rear end