Before driving it anymore, do check the trans fluid level (engine running in P) so that you aren't the cause of its failure. Assuming you still have fluid, get the leak properly diagnosed, fixed as trans leaks are far from common.
Now, as for trans failures, the major cause for trans failures in ANY 850 is primarily due to lack of fluid changes. I've seen several last past 140K miles before failing, which isn't bad considering its fluid was original but had the fluid been changes at more realistic intervals (no, not even mentioned in your maintenance book by Volvo), those trannies would still be OK IMHO. Just because it's a '93 doesn't mean that it'll definately fail unless you never have the fluid flushed periodically.