Linkage is not adjustable but I would look under there while someone worked the level to see if anything was binding. There are two rails, one for the front axle and one for the rear. Don't quote me, I could be backwards, but I think for 4lo, both shafts are pushed in the case, pull one spot back for neutral, pull one more spot and only the rear axle rail moves for 2hi, one more spot the font axle rail moves giving you 4hi. Probably some obstruction or wear in the linkage to keep that front axle rail from engaging in high range.
1986 CJ7, 4.2 w/4.0 head, TFI-HEI hybrid ignition, Clifford manifold w/Holley 390 w/cold air intake, OBA, 4.5" lift, Woody CV shaft and Tattons in front, 4.10 gears - lunchbox in front, Truetrac in the back, twin-sticked, blower upgrade for running topless, trying to keep it simple.
It's just a Jeep, and if you don't wheel it once in a while, it's not even that.