200R4 won't ****ft to 4th, TC not locking

Car is an '81 Cadillac deVille diesel.  The transmission won't ****ft into 
overdrive and the torque converter won't lock up.  This started very 
recently; before that it worked fine.  Any ideas what exactly is causing 
this, what it'll take to fix it, and how likely it is to leave one 
stranded (local, mostly non-highway driving)?

Would a 700R4 be a drop-in replacement for the 200R4?  Bell housing, 
driveshaft, ****ft linkage, etc.?  If I'm looking at major work on the 
200R4, this would seem like a good option.

Thanks for any replies.