Hello,
this is exactly the same issue I had on my TDS754D, which ended up dead as I lifted a trace trying to replace the SOJ RAM IC.
AFAIK, it almost always points to a memory error. trigNibDiag apparently fails as it cannot find something in the RAM due to a error in the memory. Did you already try to do a detailed self check?
Also, in my case, in postmortem it looks like I had a issue with a solder joint gone cold rather than with the actual RAM IS. So, maybe probing around the board a bit and/or reflowing all the relevant ISses in the first place might be workable.
Finally, beware that there are two sizes of the RAM IC. SOJ300 and SOJ400...which are incompatible.
Tam
P.S. Appended a pic of the board area for you...on mine, the problematic IS was U416.