I have an 87 K5 Blazer and want to change out the transfer case for one of the old gear drive units. Is there anything special that I have to do? I thought there was a difference with the coupling.  

Why do you want to do this. Np208's and 241 will hold up well if you have proper axle ratios behind them for tire size and usage and if you service them regularly. (you need to change fluid in them every few engine oil changes in severe duty) They cases also have a deeper low range and a planetary reduction unit which is more efficient than reduction units used in old style Tcases. I have plowed snow for many many years with chain driven Tcase and I have yet to wear out the first one. I do service them regularly and I do not run big tires with stock gearing either.  

I agree with SnoMan.  However, if you must change, then you'll need to identify your transmission.  For example, if you've got a TH350, then you need to find a NP205 that was originally behind a TH350.  You'll also need to identify your current Transfer case.  You may need to contact Advance Adapters or Novak Conversions for the proper pieces and parts.  

87 had a 208. They changed to a 241 in 88 but it looks very similar and can be bolted in.