My transmission was losing 3rd and 4th so I swapped it with on from a junk yard and I noticed when putting it in it’s missing the vehicle speed sensor doesn’t even have a hole for it and when I finsihed I took it for a drive and it was rough jerking from 40 -50 mph my obd codes where vss and oxygen sensor so I replaced the o2 sensor and now it won’t even go into 2nd and has been leaking fluid everywhere, my truck is the 1996 5.9 I know the transmission i got from the junk yard is a 98 model my question is it it possible that just both of these transmissions are garbage and I should just get the original rebuilt or is there something else that could be wrong

 

For the new transmission to be 'plug compatible' with your existing truck/PCM/wiring and accessories, you need a 46RE from a '96 or '97 truck, as those have the VSS module on them. That changed in '98 and up. Best option you have is to sell the '98 transmission you have and rebuild your existing '96 transmission.