Anything that applies even a slight load or pressure on the gears will cause then to not slide easily...and I will say here that forcing it to move when this condition exists will bent or break the shifter fork(s). They will slide easier as they break in though. With that said, in 2wd the front end is not connected to the drive shaft and bevel gear so turning the wheels should have no effect where it "may" be possible in 4wd, especially diff-locked, that some amount of additional forces may effect the gear loading. All should have the detent alignments checked and adjusted as the factory just doesn't do a good job of that or tightening all bolts and nuts. Next time this happens put your foot on the brake and shut the key off..then try it. If no change and it's still hard, it is from the drive train. But if it suddenly becomes easy, the loading is coming from the engine-side so check the RPMs and clutches.