Warning. Untested code will follow.
I think you will need lsmeans statements that set the value of pre. Something like:
proc mixed;
class group;
model post = group pre group*pre/ddfm=kr;
lsmeans group/diff at pre=val1 cl adjdfe=row;
lsmeans group/diff at pre=val2 cl adjdfe=row;
lsmeans group/diff at pre=val3 cl adjdfe=row;
lsmeans group/diff at pre=val4 cl adjdfe=row;
lsmeans group/diff at pre=val5 cl adjdfe=row;
ods output diffs=diffs;
run;
The dataset diffs should include the differences between all least squares means of group at various values (val1 through val5) of pre. Here I have variable names, but you really need the values at which you want to estimate the least squares means. I include an ODS output statement, in case you want to plot the differences and their confidence limits.
Good luck.