Rugbyjjb

so i have an 04 mustang gt manual with 73k on it. When i bought the car i noticed alittle rear-end play in the car. between gears and stepping on the gas...etc there are a few thunks and so. I had it looked at by a reputable mechanic and i was told that the rear end is pretty sloppy and that soon i would need to have some work done to it. My question is what can i do to the drive train/ rear end with a reasonable price to beef it up. I dont want to have it rebilt and it do that same to me. I have a few mods in mind for the car. Nothing crazy but i like to have fun smoking the tires etc. Anyways, could anyone tell me what exactly creates play in the rear end and what i could do to improve thanks!!! P.S. anyone have any ideas on killer gears xcept 3.73. preciate the help!