The only places you can find mini M.U.L.E.s to fix are in "Salvage Operation" missions (with description: Salvage 2 or 3 M.U.L.E.s) or in Deep Dives.
I'm not sure if you have to fix the M.U.L.E yourself or just be in the game with someone who fixes it (probably the latter), but I got this achievement easily just by playing the game normally.
The mini M.U.L.E.s normally have a green circle around them and their detached legs can be found nearby, which make a sound. Simply dig up the legs (usually 3), carry them and attach them to the mini M.U.L.E and then repair it. (Update: You can scan the mini M.U.L.E.s to see nearby leg locations on the map.)
The mission can either require you to repair 2 or 3, so if you want this achievement quickly, go for repairing 3 as it only takes slightly longer and gives you more XP.
The Deep Dive changes weekly and is random, but sometimes you will be required to fix mini M.U.L.E.s.