No, not in an imperative sentence. You are correct.
The sentence would, however, work this way:
If you would just/ only tell me, I'll make everything all right.
"Only" and "just" are not completely interchangeable. There are instances where only (or just) "just" will do, as in your sentence.
While "only" can frequently replace "just," "just" can less frequently replace "only." Of the seven meanings for the adverb "just" in the American Heritage Dictionary*:
1. Precisely; exactly: just enough salt.
2. Only a moment ago: He just arrived.
3. By a narrow margin; barely: just missed being hit; just caught the bus before it pulled away.
4. At a little distance: just down the road.
5. Merely; only: just a scratch.
6. Simply; certainly: It's just beautiful!
7. Perhaps; possibly: I just may go.,
"just" can be replaced by "only" in number 5 only.
*The American Heritage Dictionary of the English Language, Fourth Edition. Houghton Mifflin Company. 2003