2005 X-type 3.0 136,000 miles.
I am experiencing occasional transmission slippage, not so much that it has become a problem for me, but I don't want to get stranded due to sudden transmission failure. Can I expect the transmission to gradually get worse, and wait it out for another 5,000+ miles, or should I just get the transmission rebuilt now? Should I get the transmission rebuilt at a dealer, or can most any transmission shop do a decent job?

 

I'm assuming it's an automatic trans, if so you may want to try and get it serviced and add a bottle of Lubeguard Platinum trans additive. Almost all the transmission overhaul shops use this stuff as the additives can make a world of difference in shift quality and it also frees up sticking valves in the valve body. Never know, you may get lucky.

If you need to rebuild, dealer cost is going to be crazy high, so I'd recommend finding a good independent shop.

If it's a manual trans and it's slipping, you can nurse it for a while but it might not be too long before it leaves you on the side of the road.

My auto trans rebuild was just under $2k and the dealer wanted 3 times that much.

 

Transmission rebuild kit is usually about $100 +$100 converter.
It takes about 30 min to take apart and 1 hour to put it back together for experienced transmission mechanic.
Removal/installation is different.
Dealer most likely will not rebuild your transmission, they will bring it to transmission repair shop.

Ask them if they have transmission mechanic.