99 silverado running problem

my dad has a 99 silverado 2500hd with the 6.0. he brought it to the tranny shop to get a new tranny in it. the truck ran fine prior to going into the shop, however a new fuel pump about every 6 months for a year and a half or so. after coming out of shop it ran fine. all the sudden a couple days after the truck started running horribly. like it was flooding itself out after it warmed up. check engine light on for a bad o2 sensor but it didnt matter much. changed coolant temp sensor and it didnt make a difference. removed gas tank and found very milky/watery gas. cleaned tank and put fresh gas in it. ran good for a couple days(around 3000 rpms it would die out). drove it to my dads house about 30 miles away. let it sit in driveway running at an upward slant. started to drive back home and it started running like crap. about 15 mph barely and it would die out at around 1500 rpms.
found a bad wire under dash and it helped it. then it started running like crap again. dad said when the check engine light got cleared it ran a little better. and suggestions besides the new o2 sensor?

One question.. Why didn't you take it back to the place you took it tell them what you told us? Sounds like they may have tampered with something.

is it putting up a down stream o2 code? it sounds like the cats are loadin up and making it run like crap

the left rear o2 sensor was bad. i bought a useless universal one and it didnt fix the problem. the truck has 110000 miles on it. the place is an hour away from where i live. and with that truck probably 2 hours

if take a hammer and hit the cats to clean some stuff out(if they are loading up) and the truck runs better then would it just be a better idea to pound out the cats and leave it at that

You said that when the engine code was leared it "got better" , what was the code for ?? That would be the place to start.

the code was for low sensor left rear