1996 Dodge Ram truck shakes

Check engine light not on, no codes, had recently replaced transmission filter new fluids, doesn't smell of any burnt or old oil, but haven't checked the drive line or universal joints yet, I'll definitely check those. What do you think about the torque converter and fly wheel? I bought this truck as is and it looks like the transmission has been replaced perhaps off of a van or jeep? How I tell?

Well I did another complete oil change with Castrol full synthetic oil and Fram tough guard oil filter, to my amaze the truck stopped shaking after driving for a full day. I think the sensors are so sensitive the truck knew when the oil is dirty and needs changing that the mechanical parts react to the slightest change in properties of the chemical components of the lubricants, now it is purring like a cat. However, upon my quest to track down the problem before the happy moment I went to Autozone to inquire about the universal joints that this particular truck used according to the vehicle specs. Like engine size, 2 wheel drive, etc. And he gave me the OEM recommended universal joint and I took it outside to match it up, the universal joint on the vehicle was smaller than the recommended part from the store. The autozone part measured 4 1/4" across the T and 3 3/4" on the
vehicles part, something does not jive, does that mean the transmission has been replaced with a smaller one?