HELP, Locking rear diff stuck

[smileynialley]

I recently took my H2 to a garage to have the gasket between the transfer case and the gearbox changed as it was leaking.
I went to pick it up today and as i drove it away, noticed it was driving like a dog!
In drive, at rolling speed, if i put it on full lock (left or right) the car felt like it was seizing up and would only move if i throttled it a bit.
The rear wheels sound like they are slipping when i do this (turn corners) and i hear a skidding sound. the car seems to jolt a lot and to me this sounds like the rear diff is locked.
I've tried changing every setting on the console (4hi, 4lo, locking diff, etc) and nothing changes.
I've lef tthe hummer at the garage for them to sort it but do not have high hopes for them to have a clue what they are doing, seeing as they were supposed to fix a leak and ended up making things worse.
can anyone help or shed some light?
thanks
Niall