Forward only means that the shifter cam is not connecting the cam follower inside the prop shaft which means the spring that pushes the clutch dog (a sliding gear locked to the prop shaft used to engage the gears) is engaged in the rear of F gear. to get to the other two gears, you have to apply pressure to that cam follower to push it back into the prop shaft. Part way pushed back is N and fully is R.
The shaft from above comes down to the lower unit with a splined end on it. That spline is inserted in a splined (on both ends) collar which, reinstalling the lower unit is fitted on top of the shift shaft splined stub in the lower unit. The LU stub bottoms out when pushed on, but can come up out of it's mounting when removing the LU. Coming up and out of the mounting allows the shaft splines to also come out of the "shifter cam" which is the pivot and securing mechanism for the cam. With out the pivotal axle, if you will, the cam has nothing from which it can exert leverage on the cam follower and you cannot shift as a result.
Corrective action is to try your luck at fishing the shifter stub out, looking down through that small hole, lining up the cam female splines and finessing the male splines back through the cam and then wiggling all that so that you can push the shaft on through the cam and stub it in the lower mounting hole in the LU casting........lots of luck.
The alternative to that is to drain your LU oil, pull you prop, then unbolt (2) and remove your carrier bearing, then remove the prop shaft with the clutch dog and cam follower. Following that you have to fish out F gear and it's thrust bearing and shims (keep count of the shims and where you found them)......The F gear will lay flat in the LU housing and you can fish it out under the drive shaft pinion, without having to remove that. Thence you have a clear view and the access you need to hold the cam in place with a pair of long nose pliers while, with the other hand you finess the shifter stub down as I said above.
Replacement is the reverse of the removal. Before you reinstall the LU with a pair of vise grips, set the cam to the center position which is N then slide the shift rod coupler from the mid section down onto the LU splined shaft...a little lube would be inorder to help next time you remove it. Set your remote control shift lever to N. Ensure your water tube is lined up. put a small amount of grease on the drive shaft splines (sides only) excluding the top which could interfere with proper seating and if the LU won't go that last ? to 1 inch, turn the crankshaft while applying upward pressure on the LU till it eases on up
Then don't forget LU lube and you should be good to go. Sounds like a PIA but just have the engine on the boat, tilted fully up after draining the oil and it's not that bad......getting the mind set to tackle it is 80% of the job.
Good luck,
Mark