It turns out that the crappy HP procurve 1900 switch that my xenserver was connected to has a feature called “auto DOS” that was blocking the NFS packets. I installed a couple of physical Linux systems to test with and found that I could mount the NFS shares when I was plugged into a different switch.
All works as it should after turning off “auto Dos” under security settings on the switch. The crazy thing is that I had a working fog server for years on this same switch/xenserver. Time to upgrade that switch. Anyway, thanks for all your help. Much appreciated!