Good day from South Africa! Unfortunately I cannot help you directly but have two suggestions:
(1) I know that the folks from the University Presbyterian Church in Las Cruces, NM, had or even still has close ties with a church in Uganda. John Pickett was the pastor but I know that he still is associated with the UPC. I realize that Uganda is still not Tanzania but it is East Africa and much closer than us down south. I saw that Randy McMillian now is in charge of UPC East Africa Mission: see http://www.upres.com/kingdomexpansion.htm
They may bring you in contact with someone in Tanzania.
(2) You can perhaps also contact Murungi Igweta (Baptist church) through FB https://www.facebook.com/murungi.igweta?fref=ts
. He is in Nairobi. I would guess that people in that part of Africa must have some idea who you can contact.
But I also want to agree with Pilgrim that finding conservative reformed churches anywhere is a rare thing nowadays......