Nissan Repair: Pickup Transmission, best gas mileage, good mileage

Question
I have a 93 2WD pickup with a (formerly) five speed. Fifth gear quit on me, won't stay in fifth gear. Looking to replace the transmission, found a couple of five speeds on E bay, but they say they are from an 86-92 pickup. Can anyone tell me the compatibility of transmissions for Nissan trucks of various years. Curiously, since the truck turned itself into a four speed, it's gotten the best gas mileage I've seen in years.  

Answer
Robert,

If fifth gear is gone it would be cheaper to just replace the gear.  A used transmission, you describe, will fit, but you will need the driveshaft to come with it to be sure.  It will fit the engine and the frame but there might be an issue with length.  So, get the driveshaft.  But, for me, I would just replace the gear that is defective.  Overdrive, fifth gear, is only good for highway, steady state driving.  If you are in town in fifth and going somewhere between 35 and 45 miles an hour the mileage will be the same or less in fifth.  So, I think that is why you are getting good mileage presently with just fourth gear.