91 80 automatic question

[Jonas K.]

OK, so I've sourced out a 91 80 (4 cyl, FWD, auto) for my girlfriend. Her current 86 Jetta
GLI is seemingly walking that fine line of perpetual breakdown. 

Neither one of us is excited about the automatic, but she wants a "newer" car, and is fine
with moving from the top end of an economy car, to the bottom end of a luxury car. 

Question: The seller says that his mechanic said the "rear transaxle" needs replacing.
Does this make sense? I'm not at all familiar with automatic transaxle layouts, so it's
gibberish to me. The car starts fine, but when put into drive, It makes a horrible sound,
and won't move. The transmission was replaced about 2 years ago, which says nothing other
than just that. I will be doing the repair, and am looking at BTDT's on this particular
part/problem. The seller indicated that it was a common problem. Also replacement parts
costs (ballpark) for suspect components are welcome. 

TIA, 
Jonas
90 CQ