1988 chevy 5 spd trans hard to engage reverse typical fix
motor and transmission mounts. 1988 still has mechanical linkage which can be cocked aside with dirtyrotten mounts~
Make sure all the parts are clean and not worn/bent or being obstructed. Check and make sure the reverse cable is not stretched and not fully engaging the reverse gear. If the linkage is not the problem, you may have to open up the transmission. Before doing that, go to a very reliable transmission specialty shop and have a look inspection. Usually this will be free.