Have you had it scanned for stored error codes?
When was the fluid replaced last? Have you considered dropping the pan and replacing the filter?
Have you checked the mounts, front, side and rear?
If it was the torque converter, you'd get shudder on take off and downshifts. You may also feel a shudder as the TCM tries to apply the torque converter clutch after the car starts moving, about 2nd ratio.
If it's the lock up duty solenoid then you wouldn't have AWD, VDC, traction control, and most of the lights on the dash would be lit up. Same with any solenoid issue, the computer would pick up on it. If it's solenoid related, then that would require a valve body replacement.
Since you have all these miles on it, it may be something simple and fairly easy to rectify. If it is the transmission, used ones can be found relatively inexpensive. Down here in Austin I can get them for about $400 with a torque converter. Any one going this route should be sure the fluid system is clean on the car and have new fluid ready to go in, and maybe a relearn.
No one on this forum or otherwise can tell you when/if/how the transmission will fail. It's impossible to determine based on a single post. IF you have codes from the TCM or verify that you've checked physical parts external to the transmission, then it would be a little more toward a good estimation of what's happening and the lifespan.