300m transmission problems please help
 

I am the proud owner of a really nice 2001 300m with 82,000 miles.I have owned it for about 5 months...runs great ..no issues till this morning.I was cruising at about 75mph and it suddenly it acted like I had kicked it out of gear...over reved and stopped pulling.I immediately got it to the shoulder and called for a tow.While I was waiting I started her back up and shifted into drive and it took off....my joy was shortlived.Within 50 feet I was stopped again.So I had it towed to the local mechanic and of course the verdict was new transmission....my question is..is there anyone out there that thinks this tranny might not be toast.It will now pull itself a few feet in drive but no reverse...there was never any slipping...fluid is full and bright.Do I bite the bullet and buy a tranny or is there any chance something simpler is the problem?
Jerry

need to know what they think is wrong with it with out any sorta info no one online can really give any better answer.

update
 

I asked them to pull the pan....the mechanic tells me he can hear the axle turning in the transmission...I couldn't and he seems certain and kinda blew me off.I'm unsure of which way to go here...go with his diagnosis or tow it elsewhere.

Ok that still did not tell us anything