Call Standard Transmission in Fort Worth, TX. They are a TDR advertiser and a very reputable company. They rebuilt the NV-5600 in my '01 a few weeks ago at around 302k miles. They are apparently one of the few rebuilders around who have a stockpile of parts for those transmissions and will get you in and out on the road again as quickly as possible. I understand that no one is currently manufacturing NV-5600 repair parts.
Standard Transmission charges approximately $2400 for a full rebuild but if your transmission does not require a complete rebuild they will only charge you for the actual parts you need and labor to rebuild your unit.
Mine only needed the mainshaft, sixth gear, the synchro ring, and a large diameter roller bearing assembly (don't remember the part name) plus the case seals on each end of the mainshaft. I also opted for a new clutch disc and pressure plate, throwout bearing, and pilot shaft bushing because I knew family members were going to buy my truck. A shop down the street from them removed and reinstalled the transmission (I think it was a subcontractor who provides that service to them). The total bill for all parts and labor was about $1850 including the new clutch assembly. They actually installed a rebuilt unit from another truck in my truck but billed me only for the parts my transmission required.
I arrived in Ft. Worth the night before work was scheduled and stayed in a Motel 6 a couple miles from their shop. They ran me back and forth in a shop truck. The complete job required two full days. They operate like a truck repair operation assuming you need to get your truck repaired and back on the road as quickly and as cheaply as possible.
Everyone I met and talked with was friendly, helpful, and honest. I was picked up at my motel and invited back into the transmission rebuild area to be shown the parts after my transmission was disassembled and vat cleaned. The rebuild area was as clean as a hospital surgery room! The rebuilder is very experienced and willing to discuss the entire process. He showed me every component and described why it was okay or worn and why it should be replaced.
I felt that I was treated very well and very fairly. I recommend Standard Transmission to anyone in need of a rebuilt or repaired transmission.
Harvey