is that original miles on the transmission? if so i agree with 400E. i had a slow reverse in my 300se, replaced the reverse soft discs and added a thicker hard disc, thei removed the slack repairing the slow reverse, what i was getting at was, what happened to yours sounds exactly like what happens if you let the slow reverse go un repaired, there are some parts and plates and such that will eventually bend and break. see picture #119 will bend and the springs may come loose, and #107 may also crack and the alignment lug will break and the whole unit may spin or rotate causing more damage internally.