It's
very doubtful that the problem is the rear derailleur. (If it is, it probably just needs lube.)
If the RD is adjusted correctly and the hanger isn't bent (both things that the shop
should have checked), then the problem is probably chain and/or cluster wear: a new chain on the old worn rear cluster, or both just worn out. (Did your shop install a new chain?)
Take it back to the shop, tell them it's skipping under load, and ask them to check:
- RD Hanger alignment
- Chain wear
- Rear cluster wear
- That the shift cables operate freely
- That the RD parallelogram is not binding
- That the rear axle isn't bent
This shouldn't cost you since they should have been checked in the previous tune-up.
An outside possibility is a bent frame, but I gather you don't jump the bike all that much.
Ride on!
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+2
And I'd suggest you take it to a shop to get this work done - if you're not a bike-mechanic. You may need your derailleur-hanger aligned. The RD cleaned and lubed. You freewheel/cassette and chain replaced. And the wheel(s) inspected for hub-problems, etc.
If you can do this and know your stuff - Tally Ho! If not - go to a
good bike-shop.