Captial gains tax is a complex part of tax law and your situation has several variables so I'm not sure it will be possible for any of us to offer you truly meaningful advise.
I'm based on Australia and am not versed in US Tax law so I can't speak specifically to your situation. However, in Australia there are several exemptions on Capital Gains tax on inherited property. For example, inherited primary dwellings are exempt from Captial Gains Tax if they are disposed of within two years. If they are disposed of after that date, for properties acquired before 1985, the cost base is taken from the time that property is passed (i.e. the time of death of the benefactor). So this again helps reduce or avoid capital gains tax when CGT event finally occurs such as selling the property.
My advise to you would be to seek guidance from your accountant on what, if any, capital gains exemptions (or partial exemptions) may apply given that your Grandmother lived in one of the units, which means that part of the asset may be treated as her primary dwelling. You will also want to seek advise as to what date should be used for the cost base for the remainder of the asset (the part used to derive rental income). If this is taken as the date you received the asset then you may not have much of a gain at all.
Please note that I am not saying that this will apply in your situation, but rather offering you some suggestions on the sort of questions you could/should be asking your consultant.
Good luck and b.t.w. I'm sorry for your loss. Grandparents are special people worth more than any house or pile of gold (IMHO).