What's the difference between building a proforma and underwriting

I would say they are the same. But technically speaking, one may say underwriting is the act of doing diligence and making the proforma while building a proforma is just the model. 

Pro forma is just the numbers while underwriting is everything - doing diligence on the JV Partners, the market, leasing and sales comps, cost basis analysis, environmental, etc etc 

Ultimately they have the same result, a proforma (i.e. the model). If you want to be really specific, building the proforma is just modeling, while underwriting covers the entire process (research, diligence, etc.).