1994 Gmc Sierra Front Differential Question

We'll i now need to get a new front diff because every time i put it into 4wd it seems to start mking this really loud/rough clunk if i get on the gas to hard. And i brought it to a mechanic and he opened the diff cover and see the gears were pretty loose, but it only does this while in 4wd, so i have come to the conclustion, and with alittle information from the mechanic, i should invest in a new differential, and while looking around i found a few for $300 and the mechanic would put it in for $200, alittle steep but if i have to do it then i guess i have too right?

We'll my question to everyone is.. Is there an alternative to have a new diff put in , possibly somthing like a locker or a locking hub converstion setup?

If anyone has any answers to this please post or even ideas im welcoming all and any information.