Shifting problems

My 93 jeep grand Cherokee Laredo has been a beast of a ride to be roughly 30 years old. Its the inline 6 4.0 all time 4wd and It really has some get up and go power when pulling out. Not long ago I noticed while driving down the road that my baby wast changing gears. It's seemingly stuck in 3rd gear when I have the shifter in the drive position. I pull out in drive its just 1 gear and when doing about 50 mph it's sitting g at 2000 to 2500 rpm and won't go into over drive. Now you can pull the shifter down to 1 and it'll be in first, 2 second and D would seemingly be 3rd. However if your not shifting from park or reverse when you shift down to 1 then it goes into 2nd, I had a buddy tell me to pop the hood he was under there 5 seconds pushed a button on what I think is the line pressure and adjusted the cable and poof she pulled out while shifters on D and in 1st gear again like normal, and it changed gears a little late in my opinion but nonetheless was shifting again. Now the problem is back and I've tried the line pressure thing and it hasn't worked. I've checked fluid and added Lucas to it but need help as to what the big deal is. Please help me