If the bushing is bad I would expect that you could grasp the lever at the top and attempt to make is "wobble". If solid then I would look elsewhere.
The Clutch Dog spring is fully compressed for R, relaxed for F. The shifter cam/cam follower on the front of the prop shaft could be worn...from natural wear over the years.
The R interface between the dog and the R gear has vertical sides thus requiring what "racerone" said about something has to be moving when you attempt to shift to R...either the crankshaft, or the prop.
Over time the sides of the interface I mentioned get worn off, and possibly could develop burrs or smearing of the metal on one of the two surfaces, which could cause an "interference" fit between the two surfaces supporting your problem.
I think if the bushing check is OK, I'd drop the lower unit....might as well change the impeller while its off.......
I'd check both ends of the shift rod for integrity. Then with a pair of Vise Grips attatched to the shift stub protruding out the top of the LU, I'd play with the shift function selection and rotation of the drive shaft and prop shaft alternately to see if what I said about wear. If the tip of the cam follower is worn off, there won't be a definite bottom to your selection of a specific gear when you select one and rock the shift stub back and forth feeling for a positive detent.
Pulling the prop shaft/carrier bearing assembly on that engine is much easier since the redesign from Merc's original design.....remove the prop and thrust washer, undo two bolts, grasp the bearing carrier and twist and pull....out comes everything but the shifter cam which can be viewed once the prop shaft is removed.
The sides of R gears should be vertical with minimum rounding of the edges. The tip of the cam follower isn't sharp pointed but slightly rounded and the sides of the tip are defined enough to be able to slide into a shallow detent on the cam.
