1988 Sierra: Manual transmission won't go on reverse

[RayInAlaska]

A couple of days ago I noticed that the 5-speed manual transmission was getting difficult to shift into reverse, and today it won't shift into reverse at all. There is no problem shifting into all the forward gears, just reverse.

Does anybody know if there is a reverse linkage or something like that outside of the transmission, or do I have to drop it to work on it? I do have the repair manuals for this truck, but I can't tell by just looking at the pictures of the transmission.

Thanks in advance for your help.

P.S.
Could it be a "roll pin, and reverse shifter" that have caused the problem? I imagine that I would have to drop the transmission to replace both (?)