Bad rear derailleur???

Did a gravel ride on Saturday morning. 43 miles total, about 30 was gravel.. Lot of rough gravel, very few hard, smooth, compacted stripes on the gravel road. I'm accustomed to gravel roads having well worn tracks where the cars drive. Nope.

For the final 6 miles of pavement, the rear derailleur did not work. When shifting up and down the cassette, it did not want to find the cog and jumped around a lot. I'd click and nothing happened. I'd click again and nothing happened. I'd click again and it would sort of grind and jump around looking for a cog. So I'd have to shift a bunch to get it to settle down into one cog. It did not go thunk into the next cog when shifting the STI levers.

And when shifting down to the small ring, the chain drooped on the bottom. The spring in the rear derailleur to pull it backwards and take up the chain slack was not working correctly. It wasn't strong enough to pull backwards with enough force. Had to ride the big ring only for the final miles, with the bad between cog shifting too. I basically had a single speed.

I cleaned the bike with the garden hose afterwards thinking maybe it was just the gravel dust that ruined everything. Have not had a chance to test the shifting in the basement shop. Will also try taking the derailleur off and re-lubing everything and really cleaning everything with gasoline. And spraying lubricant into the STI levers. And oiling the cables.

Questions: Can any 10 speed Shimano rear derailleur work? Dura Ace, Ultegra, 105, Tiagra, AND all of the mountain bike rear derailleurs? Nashbar has a lot of 10 speed rear derailleurs for sale. Thinking maybe a mountain will be better than road for gravel and mud? Its a short cage rear derailleur. But I was thinking maybe going medium or long if I could. Yes, no? I've never noticed any shifting difference between derailleur lengths. Current rings are 46-36 with 12-28 cassette. But maybe I will want to go 46-33 rings (110mm bcd five arm crank) and 11-32 or 11-34 ten speed cassette. Make it my second mountain climbing bike. But its a heavy steel frame...