Drive Shaft ???
 

Sawpped from a 10 to a 12 bolt in my 84 SWB 4x4. Didn't think to check drive shaft length, looks like I have ALOT of yoke hanging out of the transmission. 12 Bolts shorter/need longer driveshaft?

Re: Drive Shaft ???
 

I did that same swap about 15 years ago in a mid 80's blazer. I used the same driveshaft, but I had to get a conversion u-joint as the rearends had different yokes. If you still have the other rearend you can always measure the distance from the yoke to the centerline of the rearend on both of them just for a comparison. I think it should be close enough to not make a difference in your drive shaft length.