What trans do I have?

[pofarm]

My truck is a 1991 K2500 with a 454 engine. It has a 5 speed manual with overdrive. I was told that it is probably an NV4500, but all info on the net says that GM didn't start using this trans until 1992. My truck was built in 7/91 and uses a 92 front drive shaft, though. I found this out when I replaced the U-joints in it. Could my truck possibly have a 1992 trans, also?
The trans has problems. The bearings are noisy and it grinds going into 3rd gear. I am hoping to find a good used one, hence the reason for my question.
Also, I would like to go ahead and change the lubricant in it. I need to know what to use, depending on what trans it is.
Thanks!