How do I create a probe for in situ hybridization?
I dont understand what you mean by hydrolysis in this context. 

The way to go about is:


Clone the DNA, that will give rise to the probe, downstream of T7 (or any other) promoter
Linearize this plasmid by digesting at (or ahead of) the 3' of cloning site
Set up an In-vitro Transcription reaction with T7 polymerase (or whatever corresponds to the promoter you used)
Precipitate and purify the RNA (you can use DNAseI to get rid of the DNA).
To check the RNA size: mix ~2$\mu$l of RNA (less or more depending on your yield) with equal volume of 2x RNA loading dye (contains formamide 95%, EDTA, SDS, and the dyes). 
Heat at 85⁰C for 5min. Let it cool and run in 1% TAE/TBE-agarose gel (no need to make MOPS-Formaldehyde gel)
dsDNA ladder wont be very accurate but may be used for size estimation (if linear, the mobility of dsDNA and ssDNA/RNA wont be very different. Formamide treatment keeps the RNA linear). Use ssDNA or RNA ladder for better accuracy. 


Btw.. the absorbance ratios mentioned are okay.. you don't need an RNA of supreme quality for this application (in fact this suffices for most experiments)