First of all, the slope variance estimate from PROC MIXED is for the subject slope "effect". You would need to compute the variance for ssdev rather than sscoeff with PROC UNIVARIATE if you want to check on the consistency of the variance estimate.
Second of all, you are not going to get the same variance estimates by these two approaches. The SSDEV estimates are BLUPS and have the "shrinkage" property. The variance from PROC UNIVARIATE should be close, but is likely to be smaller, and that is expected -- the two variances are not the same thing.
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